HI!

Yes you can change just callsign in preferences/station.
That leaves just the date of old qsos, that only you know, as separation between new and old callsigns worked qsos.

I suggest following:
- open cqrlog and your current log
- go "QSOlist/File/export/adif" and do full adif export (all checkboxes selected)
- go "newQso/file/Open or create" new log. If "Open recent log after program start" is checked uncheck it.
With your current log selected (blue background) there goto "utils/configuration/export" and save your log's configuration to file.
After that select "new log" and give a name and number for your new log.
When return you should have a new log in list with blue background (as selected)
go "utlis/configuation/import" and import log settings you just saved to file
- open new log with button "Open log". If this fails (there may be some unfixed bugs) then close cqrlog and open it again with your new log.
- goto QSOlist window and select "File/import/adif" and import qsos you did just export to adif file
- at this point you should have 2 similar logs your old one (as backup) and the new on you are going to use with new call.
- When new log is open go to "preferences/station" and change your callsign
- Next go "QSOlist/File/Group edit". Select "field" as "comment to qso" and type to "value" "qso as YourOldCallsign". Press "Apply"

If you have made comments for old qsos they are still in original old log. Comments in this new log will tell you did you work qso with your old call.
Start using this new log with your new callsign.