Hello,
I've upgraded from 2.4 to 5.0.9
Upgrade to port the server from Debian wheezy to Debian buster.
Algorithm:
1) dump database on server_old
2) installing Zabbix 5.0.9 on Server_new with a new empty reference database
3) creating a new empty database on the Server_new server, importing data with a script
# cat /root/zabbix_import.sh
#/bin/bash
MUSER="zabbix"
MPASS="zabbix"
MDB="zabbix"

LIST=$(ls /root/full/)

cd /root/full/
#echo $LIST

for i in $LIST
do
{{ echo "import $i"}}
{{ zcat $i | mysql -u$MUSER -p$MPASS $MDB}}
{{ echo "import done!"}}
done

4) During the import process, the host_inventory table could not be imported, due to an error
{{Row size too large (> 8126). Changing some columns to }}
{{TEXT or BLOB may help. In current row format, BLOB prefix of 0 bytes is stored }}
inline.
It was solved by adding the line SET SESSION innodb_strict_mode = 0; to the data import script

5) zabbix did not start. According to the logs, there is a problem with the encoding. Changed encoding using utf8_convert.sql procedure
DELIMITER $$
CREATE PROCEDURE zbx_convert_utf8 (
)
BEGIN
declare cmd varchar(255) default "";
declare finished integer default 0;
{{declare cur_command cursor for }}
SELECT command
FROM
{{ (/* This 'select' statement deals with 'text' type columns to prevent}}
{{ their automatic conversion into 'mediumtext' type.}}
{{ The goal is to produce statements like}}
{{ ALTER TABLE zabbix.hosts MODIFY COLUMN description text CHARACTER SET utf8 COLLATE utf8_bin not null;}}
{{ */}}
{{ SELECT table_name AS sort1,}}
{{ 'A' AS sort2,}}
{{ CONCAT('ALTER TABLE ', table_schema, '.', table_name,}}
{{ ' MODIFY COLUMN ', column_name, ' ', column_type,}}
{{ ' CHARACTER SET utf8 COLLATE utf8_bin',}}
{{ case}}
{{ when column_default is null then ''}}
{{ else concat(' default ', column_default, ' ')}}
{{ end,}}
{{ case}}
{{ when is_nullable = 'no' then ' not null '}}
{{ else ''}}
{{ end,}}
{{ ';') AS command}}
{{ FROM information_schema.columns}}
{{ WHERE table_schema = @ZABBIX_DATABASE }}
{{ AND column_type = 'text'}}
{{ UNION}}
{{ /* This 'select' statement deals with setting character set and collation for}}
{{ each table and converting varchar fields on a per-table basis.}}
{{ It is necessary to process all tables (even those with numeric-only columns)}}
{{ otherwise in future Zabbix upgrades text (e.g. varchar) columns may be added}}
{{ to these tables or numeric columns can be turned into text ones and}}
{{ the old character set/collation can reappear again.}}
{{ The goal is to produce statements like}}
{{ ALTER TABLE zabbix.hosts CONVERT TO CHARACTER SET utf8 COLLATE utf8_bin;}}
{{ */}}
{{ SELECT table_name AS sort1,}}
{{ 'B' AS sort2,}}
{{ CONCAT('ALTER TABLE ', table_schema, '.', table_name,}}
{{ ' CONVERT TO CHARACTER SET utf8 COLLATE utf8_bin;') AS command}}
{{ FROM information_schema.tables}}
{{ WHERE table_schema = @ZABBIX_DATABASE) s}}
/* Sorting is important: 'MODIFY COLUMN' statements should precede 'CONVERT TO' ones
{{ for each table. */}}
ORDER BY sort1, sort2;
declare continue handler for not found set finished = 1;
open cur_command;
cmd_loop: loop
fetch cur_command into cmd;
if finished = 1 then
leave cmd_loop;
end if;
SET @value = cmd;
PREPARE stmt FROM @value;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
end loop cmd_loop;
close cur_command;
END$$
DELIMITER ;

script usage
MariaDB [mysql]> use zabbix
MariaDB [zabbix]> SET @ZABBIX_DATABASE = 'zabbix';
MariaDB [zabbix]> CALL zbx_convert_utf8();
MariaDB [zabbix]> set innodb_strict_mode = ON;
MariaDB [zabbix]> drop procedure zbx_convert_utf8;

zabbix started up and works. But the logs periodically have errors
6) compared the structure of the resulting database with the database created during the installation of a clean installation of zabbix 5.2 fixed minor differences in structure.

Now zabbix works, but sometimes errors appear in the logs.
zabbix_server [1223]: ERROR file:dbconfig.c,line:10344 Something impossible has just happened.
{{ 1223:20210324:112438.718 === Backtrace: ===}}
{{ 1223:20210324:112438.718 13: /usr/sbin/zabbix_server: trapper #3 [processing data](zbx_backtrace+0x3f) [0x56163c2321be]}}
{{ 1223:20210324:112438.718 12: /usr/sbin/zabbix_server: trapper #3 [processing data](+0x1adc62) [0x56163c20dc62]}}
{{ 1223:20210324:112438.718 11: /usr/sbin/zabbix_server: trapper #3 [processing data](DCget_status+0x4a) [0x56163c20e2dd]}}
{{ 1223:20210324:112438.718 10: /usr/sbin/zabbix_server: trapper #3 [processing data](+0x81bba) [0x56163c0e1bba]}}
{{ 1223:20210324:112438.718 9: /usr/sbin/zabbix_server: trapper #3 [processing data](+0x81f41) [0x56163c0e1f41]}}
{{ 1223:20210324:112438.718 8: /usr/sbin/zabbix_server: trapper #3 [processing data](+0x829af) [0x56163c0e29af]}}
{{ 1223:20210324:112438.718 7: /usr/sbin/zabbix_server: trapper #3 [processing data](+0x82edc) [0x56163c0e2edc]}}
{{ 1223:20210324:112438.718 6: /usr/sbin/zabbix_server: trapper #3 [processing data](trapper_thread+0x276) [0x56163c0e3176]}}
{{ 1223:20210324:112438.718 5: /usr/sbin/zabbix_server: trapper #3 [processing data](zbx_thread_start+0x37) [0x56163c240f15]}}
{{ 1223:20210324:112438.718 4: /usr/sbin/zabbix_server: trapper #3 [processing data](MAIN_ZABBIX_ENTRY+0xb2d) [0x56163c0a974f]}}
{{ 1223:20210324:112438.718 3: /usr/sbin/zabbix_server: trapper #3 [processing data](daemon_start+0x2ff) [0x56163c231dca]}}
{{ 1223:20210324:112438.718 2: /usr/sbin/zabbix_server: trapper #3 [processing data](main+0x2f5) [0x56163c0a8bd7]}}
{{ 1223:20210324:112438.718 1: /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xeb) [0x7f01d33ee09b]}}
{{ 1223:20210324:112438.718 0: /usr/sbin/zabbix_server: trapper #3 [processing data](_start+0x2a) [0x561}}

{{ How to resolve an issue with errors}}