I think you better re-read the requirements for Rapid-PVST+. 3500XL series switches don't support it so connected devices running Rapid-PVST+ will fall back to PVST mode anyway. In other words the upgrade will do nothing for you so long as you have the 3548XL switches in there.
As for the downtime question, there will likely be a disruption since you would be building an all new tree with the different STP version.
Depends on so many factors that no one can answer this I'm afraid.
If the bridges you are migrating are just access bridges and the rest of the network is running rapid-pvst, the host attached to the switch you are migrating will experience some traffic disruption for few seconds probably.
If the distribution is running PVST, add a good 30 seconds to that (and be careful about how inefficient stp/rstp interaction is).
If the bridge you are upgrading is not an access switch, meaning that there are other bridges downstream connected to it, it becomes much more complex to evaluate how long it will take. What is your expectation?
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