C5 probably won't change much, but if they wanted to use the power of CBS to relaunch it I think closing it and replacing it with a CBS branded channel would be the way to go. Frankly there are probably only 4 brands on C5 worth keeping - Neighbours, Home and Away, Jeremy Vine and Milkshake. Beyond that it is a clean slate.
I think it's highly unlikely that ViacomCBS would interfere with the Channel 5 brand in the UK, in the same way that they have Network 10 in Australia.
The channels that seem at risk are the CBS branded ones in partnership with AMC Networks International such as CBS Reality, CBS Justice and CBS Drama, plus the Horror Channel. However, I think there's plenty of room in the UK market for these alongside the Channel 5 network.