The warm restart is sufficient for restarting the collector and picking up the new limit.
I'd suggest log a support case with fresh irfax and the new defsrec file for our investigation.
For others to know - reply from support
The size of a STRING field in a record must be less than 255 bytes, i.e. there is currently no way to extend it to a bigger size.
However, there might be a possibility for you by using EMS Insight for HPE NonStop https://help.prognosis.com/server/119/infrastructure/ems-insight-for-hpe-nonstop
Please note, that a separate license is needed for using EMS Insight.
I have therefore added your Account Manager to this email who can discuss this option with you.