What I've always heard/been told is that you want to use a lamp right away to make sure it's not DOA, or defective since lamps usually only have a short, eg 90 day, warranty (I'm speaking in generalities here not specifics to any brand). On the other hand lamps don't have a shelf life, or wear out when not in use so they can be stored indefinitely.
Thus good practice is to use the new lamp right away and keep the older working lamp as a backup. That way you make sure the new one works, while having a "known good" lamp as a backup.
Even if JVC's warranty is better (I haven't followed) I'd think you still might want to use the new one right away so you get the most warranty coverage out of it rather than running your old lamp which is likely out of, or closer to out of warranty.
Other option is to try the new lamp, and after you're sure it works, swap them back.