Sure you can replace just the threshold.
It may be worth removing the door to inspect for more rot though, guess it depends on whether there is evidence of any rot in the framing. If so not a big deal to pull the door out, make sure all issues are addressed.
It may be worth removing the door to inspect for more rot though, guess it depends on whether there is evidence of any rot in the framing. If so not a big deal to pull the door out, make sure all issues are addressed.