I'm planning to setup a Hyper-V virtual machine with Windows 10 and try to migrate some business applications. If everything works, I plan to activate with a Win 10 Professional license key. Unfortunately, I only see the Enterprise iso available for download.
Is it possible to switch the version later during activation?
Or can I get an official Professional evaluation ISO somewhere?
Unless they have changed practice you won't be able to activate an evaluation with an OEM/Retail key anyway. The licence type of the media and key must match https://social.microsoft.com/forums/en-US/44179f86-f8a6-4dc2-8692-b1637e72280b/windows-license-types-explained
You are expected to trial run then reinstall with valid licence/media for actual use. If you make good documentation as you test migration of your apps, installing windows and doing the process clean shouldn't be very much more work and likely to end up with a neater more reliable result.
You can do a repair install over the top as in https://serverfault.com/a/173559/15008 to change types but that can get messy, I would only do it if you are desperate to keep something and with a backup in case it goes wrong.
I just went ahead and downloaded the Win 10 iso (the normal one, not the evaluation), and started to install it in Hyper-V.
It looks like it is possible to do the installation without a key and activate later: