You should be able to do it relatively quickly, We take less than half a day for this to run. This depends on complexity and might be different if you are running a huge grid. In a development environment you can definitely test though and ensure you're accounting for what you have in parallel. We are lucky to have a DEV and VAL environment where this is thoroughly tested. That effort takes weeks for us to really make sure we are ready for that switch.
One thing to mention is for our installation for 9.4 M7, we have taken down the M7 Mid-Tier due to the huge number of security vulnerabilities associated with those components. We are glad M8 is out!
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