A tubular can be changed in about 2 mins by an experienced user.
When you put on a spare it should have a coat of glue on the base tape and it should be folded base tape to basetape in order for the glue to stay tacky.
The tacky glue reactivates the layers of glue on the rim and thus provides for a decent bond.
I'd never consider this safe enough to push this setup to the limit, more like safe enough to carry you home.
When I get home I put on a new tubular but that's just because I use an old one with few mileage left as a spare.
Besides that I can't be bothered to center a spare to perfection when I know I'm just putting it on for one ride back home.
Should you use a fresh tubular as a spare then chances are you'll still have to put it off the rim again anyway as you'll likely won't have it centered the way you'd prefer it and want to make sure you have a perfect bond again.
No need to restart the entire gluing process again, a new tubular will be happy with just a coat of glue on the basetape.
The rim receives just a thin coat of glue again and after a few minutes the new tubular can go on the rim.
Should you notice the old layer of glue has become very patchy and uneven then now is a good time to clean the rim and rid it of old glue.
In which case you prep the rim as if it were a new one and go through the entire process.
This is something I carry out maybe once every other year.
The better the original glue job the less chance for it to deteriorate with every removal of a tubular.