Should I disable the Avast shields before I update?
Yes.
+1
I shut down all programs that scan activity prior to doing the update.
(Avast, Winpatrol, Malwarebytes, etc, etc. If it scans, it could cause a problem and could possibly block something vital.)
I updated 2 systems yesterday (one 32 bit one 64 bit) following those rules and had no problem with either system.
This will also speed up the process which is a long one.
Whilst this is a reasonable precaution given the complexity of this update. I would advise all considering doing this that they make a full system backup.
Two reasons for this:
1. an OS update can have its own issues and mess up your system.
2. being on-line for what may be an extended period without antivirus protection could result in getting infected.
Note two above, this happened to me many, many, years ago before installing avast (about 13 years ago) having just installed XP Pro and the CD didn't have SP1, going on-line to install that resulted in an infection. Fortunately I had drive image backups so not a disaster.