When editing regular expressions associated to a Low Level Discovery item, items are not create nor deleted with that change.
I believe this should be part of the process of discovery.

Steps to reproduce.:

• Create an LLD item for SNMP ifAliases which has the {#SNMPVALUE}

  with the filter @Network interfaces for discovery.

• Associate this template to a host and let it be discovered.
• You edit the @Network interfaces for discovery and add in 3 » Bearer Channel [Character string not included].
• Items, triggers and graphs aren't removed based on that filter.
• Unlink and Clear the template from a host, save, then re-add, Zabbix fetches only the correct items, but loses all history and trends.
• Remove 3 » Bearer Channel [Character string not included] but Zabbix won't re-discover the items.
• Unlink and Clear the template from a host, save, then re-add, Zabbix fetches all the items, but loses all history and trends once again.

I done this test with SNMP, but I believe it's the same with the Agent.