check the keyboard layout settings - that's the usual problem here. Your upgrade is setting the keyboard to something incorrect, like switching from UK to USA etc. You can prove this is the problem by simply typing the password into the user name box and seeing if it comes out right.
Then, check which pre-boot keyboard layout is selected and correct if needed.
Thanks for the reply. That was the first thing I checked. We had the issue where Safeboot would slip into UK English and # sign changed to the UK PD symbol. The upgrade only had 1 check box for both US and UK english. I also checked the options at the signon screen and both were correctly set.