I saw this same issue reported recently, so it may be a bug. If you have not done so already, I'd suggest reporting this to Tableau support.
Please also take a look at the following. It includes a workaround that may help you: Editing Data Connection in Tableau Prep hangs
Sorry to hear you are running into this issue. It definitely sounds like a bug - you should be able to directly edit a connection and point to a different datasource without the app hanging or crashing. Could you please file a support case so we can follow up with you and get more details - this'll really help us identify the problem and get a fix out.
I've opened case number 04016604 for this problem. I ended up remaking the entire flow on a new data source (which was time consuming and not repeatable). I'd like to repeat this flow for new months of data without recreating the steps if possible.
Did you happen to try the workaround I shared above?
Yes - I tried adding a new source and connecting 'instead' of the existing one, but to similar freeze-up results. I also tried just deleting a connection and then adding a new one but had similar dismal results.
I'll upgrade to 2018.2.1 and see if that corrects it.
Too bad. Good luck! Give Prep a chance. It's new and all these kinks will be worked out very soon I'm sure.