hello i currently have a 50-34 11-34 set up. I want to know if I can just swap out 11-34 with a 11-30 without changing anything else besides the cassette?
Yes, swap away. I use a Wahoo Kickr and swap cassettes without needing to set anything in Zwift.
ah my bad i didnt say it was for my bike not my trainer.
You will need to adjust b limit at minimum. If your chain length is setup correctly with the 34, it may well need a link removed otherwise you risk your chain being too slack. Best to check chain length AFTER installing new cassette and adjusting B limit as it might well be fine. This applies to both trainer and bikes.
The Shimano chain sizing method, often called “big-big-plus 2”, is to run the chain on the largest chainring, the largest cog, no derailleur, and add 2 links (hence the name). Changing from 34T to 30T would probably mean taking 2 links out. This said, the drive train may work very well without shortening the chain, in particular if you have a longer cage derailleur.
The Shimano sizing method bypasses the derailleur, so B limit screw adjustment would take place after the chain size is set.
Shimano manual is not what bike shops follow as it can be needlessly costly for customers 
It’s also the method Park Tools recommends. And Sheldon.
Not sure how chain sizing can influence the cost…
Oh dear - Remove links, move bike off trainer back to road wheel. Oops, too short. Rejoin links… amateur hour.
2 seconds to set B limit without destructive link removal. The internet isnt always the smartest!
I assume you missed this part.
There’s nothing destructive in removing chain links.
