Is it easy to turn W8/8.1 SL into W8/8.1 Pro?

Situation is....

Company wants to buy closed boxes that come with Win 8.1 Single Language.
Would it be easy to, later on, turn these on into Win 8.1 Pro to join a domain, since Single Languange boxes cannot join domains?

Or would it require the complete OS clean install to do this?

Or a simple key change would change the Windows edition, when assigning to Microsoft Channel?

Thanks for answers...

Would require a New Product Key

Yes, so, to make the machines join a domain, is it only a matter of changing the Product Key given by the volume licensing channel, for example, and SL turns into Pro automagically, after the validation of the key and activation? So it is that simple? No need to clean OS install?

Should be doable. I've done it with 8.0 Core to 8.0 Pro, I just used add features and entered my Pro key. As long as its a legal valid key that is. When you do the add features it upgrades, then reboots, then checks activation. If you use a bogus key it will still upgrade but fail to activate.