The form version is a statement of what "version" the record structure follows. SSE has a different record structure from Skyrim.
New Records in SSE
LENS
VOLI
SPGD (from FO4)
Changed records in SSE
MATO - changed Directional Material Data (L9895-L9929)
STAT - changed Direction Material (L12783-L12797)
WATR - changed Visual Data (L12920-L13041), added Flow Normals (L13057)
WTHR - added volumetric lighting (L13426-L13432), added LENS flare reference (L13442)
WEAP - changed Critical Data (L13159-L13183)
Changes subrecords in SSE
VMAD - added "large references"
Overview
If the plugin includes any of the above records or subrecords it NEEDS to be updated, else unknown/unpredictable problems may occur. I'd recommend converting ALL plugins though, as there may be other changes that have not been decoded yet.
It's your call, but why risk your save games/game stability when you can quickly and easily convert plugins to Form Version 44?
Source: Changes to TES5Edit wbDefinitionsTES5.pas since SSE was released
This applies to porting mods from LE to SE but all of the games likely have a range that forms should be in.
This applies to porting mods from LE to SE but all of the games likely have a range that forms should be in.