The short answer is yes; the long answer is yes.
Multi-installation – where you have MQ V9.0.0.0, MQ 9.0.0.1 and MQ 9.1.0.0 on the same box – I would have called this multi-version or multi-level support. This is not to be confused with multi- instance queue managers are where you have primary and backup queue managers running on the same level of code.
Without multi-installation
When you migrate to a new level you have to shut down your queue managers, delete your old level of MQ libraries, install the new level of MQ ones and restart the queue managers. If you want to go back you have to reverse the process. This assumes you have the installation materials for the previous level, and all of the fixes you may have applied to it. All of this takes time (and you may spend a long time looking for the CD because your corporate network rules do not allow you to download big files). During this time your queue managers are down.
If you have more than one queue manager on the image, they will all use the new level of code when they restart.
With multi-installation
You can install new levels in parallel, run a command and a queue manager will use the new libraries “at the flick of a switch”. So the actual migration is shut down MQ, issue the command, restart MQ. If you have more than one queue managers, you can migrate one, then next day migrate the others.
Be careful…
The MQ documentation says
For example, if you want to upgrade IBM MQ Version 9.0.0.0 to IBM MQ Version 9.0.0, Fix Pack 1, you can install a second copy of IBM MQ Version 9.0.0.0, apply the maintenance bring it to IBM MQ Version 9.0.0, Fix Pack 1, and then move the queue managers across to the new installation.
You still have the original installation, so it is a simple matter to move the queue managers back if you encounter any problems.
I think this is carefully worded. 9.0.0.0 to 9.0.0.1 and back to 9.0.0.0 may work as the documentation says. I do not feel confident that you can go from MQV8 to MQ V9.0, or MQ V9.1, run for a week, and then be able to go back. I would love to be proved wrong.
ColinPaice 
Hi Colin, Yes, you can roll back a fix pack so 9.0.0.0 up to 9.0.0.1 and then back again is possible. You cannot roll back a release though, so 9.0.0.0 up to 9.1.0.0 cannot go back again. Still handy to have multi-version install and have the two releases in parallel for testing purposes, even if a specific queue manager cannot go backwards (except with a backup and restore – which takes it back in time too!)
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