Going by the picture, I'm seeing you're going from a carbine to a rifle stock(by looking at tube and buffer). Yes, you'll definitely need to change out (all) the components to make things work/assemble correctly.
As others have said, you'll need a castle nut wrench and putting the lower into a padded vise to break the nut loose in most instances if the nut was torqued on correctly. Also, you may have staked metal in the nut's locking grooves which will make getting the nut off harder--but not impossible. Watch for that spring slipping out unexpectedly behind the end plate like Hillbilly mentioned.
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