1. I had a host macro "{$PROC_KERNEL_COUNT}" with a value (e.g. 200)
2. I loaded https://zabbix.garnercorp.com/hosts.php#macroTab for the host.
3. I changed that field to "{$PROC_KERNEL_COUNT_OLD}"
4. Click Add
5. Enter a new macro "{$PROC_KERNEL_COUNT}", with a value (e.g. 250)
6. Click Update

Actual results:

Cannot update host
Details
Macro "{$PROC_KERNEL_COUNT}" already exists on "hostname".

Expected results:
treat the set of operations as an atom, don't try to validate changes individually, validate the set.

Workaround:

1. Rename the field
2. Update the host
3. Editing the host again
4. Add the field with its original name
5. Update the host