Running 2100+ going to downgrade to k7 1100...no post?

I have a computer I built for my grandmother using a 2100+. I came across a good deal on a K-7 1100 and want my 2100 back so I installed the 1100 in her computer and it didn't post. Is there something that needs to be done to install a slower CPU in a comp? The motherboard is a GA-7VAXP Ultra and according to Giga-Byte the K7 is supported and the multi is set to auto and and the CPU clock switch is also set to auto. Any help would be great, thanks.