I bought a PowerBook 540c on eBay, because I wanted a 68k laptop. When it arrived, I discovered it had been upgraded to PowerPC.

I have two questions:

1. Would it be possible to downgrade this Mac by buying a 68k CPU or motherboard? (I'm not sure how much hardware the PPC upgrade changed.) There are a few of each on eBay.

2. What is the earliest OS that the PPC-upgraded PowerBook 540c could run? It's currently on 8.1 — will I run into any problems if I try installing an older system?

I have a set of original floppies that came with the 540c, which I suspect contain System 7.1. I imagine they won't work, because the CPU upgrade is newer than the floppies, correct? 

I'd love to get 7.5 on there if I can.

Thanks for your help,

Amy