Hi - I am trying to DSTNAT from internet to local RouterOS web interface, I simply want to change to external port for the Web UI but leave the internal port. ie. DSTNAT 80443 to 443 on device. Is there any way of doing this?
It’s completely impossible, I’m afraid. You’d have to redefine tcp’s ports, have everyone rewrite their software, etc. It would take many years. But if you’d settle for port number 65535 or lower, it’s dstnat rule like any other. 
You can even simplify it a little bit, use action=redirect and you won’t have to enter target address. But it will be new connections from WAN, so you’ll have to allow them in firewall’s input chain.