Are there any major difference in sound btn universal and custom?
Yes there is. By how much depends on individual. Some ppl experience huge differences as a good seal in universal is not exactly the same as a good seal in customs. Don't know if that makes any sense to you
Some ppl who auditioned Roxanne had some complains about the sound but when they received their customs the issues were absent. Same goes for other brands ppl experience demos diff from the actual customs. But diff by how much only you'll know.
If it just the fitting then is it a good idea to get the universal first and maybe or maybe not reshell sometime later?
By reshelling, the sound may change. Furthermore JHA do not do reshelling so you have to look at other alternatives.
Price difference is more then 1k btn universal and cf custom.
If you go for acrylic instead of CF will save you $500 USD