Safely Switch Bicycles before a Century? I did an unsupported century last year on my 30 pound "shop" mountain bike.  The first 55 miles were easy but the last 45 or so were pretty horrible. 

This year, I want to try it again but with more hills. I really don't want to lug my 30 pound mountain bike up the hills, but I don't have the money for a decent road bike.  

If I follow a reasonable training plan on my 30 pound mountain bike this year, will I be "safe" moving into high-dollar 15 pound rental road bike 24 hours before an unsupported century?

WARNING: This question was edited from its original version. It originally used the term "homemade" century to denote an unsupported century.  Also, the question originally asked if it would be okay to switch to a road bike the day of the mentioned century.  Now, the question asks if it is safe to switch 24 hours before the mentioned century.
I would strongly advise against that. Your legs are not going to be the problem, and neither is your overall conditioning/strength.

If you are going to be around other riders, you need to be able to stop and start and steer safely. Road bikes steer with your butt while MTB steer with handlebars.  This is a big deal when you are tired and running on instincts. You don't want to crash or cause a crash.

Your core and arms are probably not going to be strong enough to hold you up for a century on a road bike unless you've trained on one. A spin bike could help if you can get a road-bike-position. Also doing lots and lots of planks. Think of riding a road bike in a century like a 4-5hr plank. If you can't hold it that long, you'll start to sit which causes pain in your rump and arms/hands.

Short story:

Ride what you trained to ride, in this case the MTB. I often feel 'whiny' at the 45 mile point and again around 60 miles and again at around 90 or so. Usually eating something fixes it (power bar in my case). Almost like someone lubed my knees or something and I'm raring to go again.  I usually only stop long enough to refill water bottles and carry all my food on the bike.