How do I enter diagnostics mode on Xerox Phaser 3635MFP?

SOURCE: Phaser 8500DN

Grrrrrrrrrrrrr. Do you have a maintenance agreement? If you do call them back and tell them to get there asap.
Sounds very much like the firmware is corrupted. If you cant get to diagnostics, here is what you can try.
On the back of the machine open the panel to access the memory, remove the memory and power the machine up with the memory removed. Let it fault out, then replace the memory and start it again. See if that works.
Then remove and reseat the main board by turning the orangish knobs and sliding the board out and back in.
It really sounds like the upload corrupted and Xerox needs to resolve this for you.
They will try and say it's not their fault, but if they directed you to do the upgrade, they are at fault and should fix it for you.
Unless, and I very much doubt the old software is on their site, no you cant go back to the old firmware, you may try reloading it and see if it loads correctly.

SOURCE: xerox 5765 enter diagnostics

any 9 code is xerographic. This machine isn't worth fixing if you want me to be truthful. The parts alone will cost you more than enough. Look for DC12 on ebay or something to upgrade.
Sorry, but its the best solution

SOURCE: Xerox phaser 8550 prints garbled characters

This happens to mine occasionally after I have put it into standby a few times. If I reboot this always sorts it out.

SOURCE: Xerox DocuColor 1632 /2240 Enter the Diagnostics Mode

fuji xrox 2240 error code 102-372

SOURCE: Xerox Phaser 6100 vs Windows 7

Try using the Vista driver, it may work and may not have all the functions that was available under Vista.