Personally, I don't see an issue. The only advice I would have is to keep RoboHelp 8 handy as well as a good backup. Just on the outside chance we encounter a bug that we didn't anticipate. That way you could easily revert if you needed to.
But I'm not seeing anything suggesting that a bug is looming. I've moved all my production to RoboHelp 9 without issue. Seems to be working fine. At least for me.
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I would imagine so - how does RH know if you're mid-project? A project is just a project to RH when it converts it. Just make sure you've got a backup for safety ;>)
Once you open a project in RH9 there is no going back. A message is issued warning you of this when you upgrade. For this reason it is better to take a backup of your RH8 project before doing anything. As for whether it is OK to upgrade to RH9 mid project, we can't say because we do not know what is inside your project and how you are using it. Personally I would never upgrade mid project, but that is just me. I recommend you download the RH9 trial, take the required backups, open your projects in RH9 and thoroughly test them before making any decision.
The RoboColum(n) @robocolumn Colum McAndrew