Changing cassette advice

[Daninplymouth]

@Daninplymouth
I misinterpreted your OP.
New bike = new 11-34 and new chain (say 56")
Old bike = worn 11-30 and worn chain
New wheels
"plan would be to swap my new cassette to the same 11-30 would this simply be a case of swapping the cassette "
So is the "same 11-30" the one you have currently on the old bike or is it 'new' (in which case what is it "the same" as)?
Either way, if the chain (on the new bike) is the right length for the 11-34 then it will be longer than required if you fitted an 11-30 cassette. Large/large the chain is engaged from 190o round to 352o on 16 of the the 34t (= 8") and on the 30t sprocket it'd be 7": one inch less chain required. Benefit: less 'slap' in small chainring and smaller sprockets on bumpy ground/roads eg the Plym Valley cycle path up to Yelverton

Thanks yes it would be a brand new cassette. So it’d be best to get 1 link removed from the chain toO

 

[Ajax Bay]

Have suggested that, but refer to my first post and measure the nearly new chain in that way round the 50t and the 30t. No point in relying on the chain being the correct length now (for a 11-34 cassette) and just blithely removing one (") link.