A very basic Static PAT / Port Forward on the new software could be something like this (though I personally use Network Object NAT for this)
object network HOST-REAL
object network HOST-MAPPED
object service RDP
service tcp source eq 3389
nat (inside,outside) source static HOST-REAL HOST-MAPPED service RDP RDP
access-list OUTSIDE-IN permit any object HOST-REAL eq 3389
access-group OUTSIDE-IN in interface outside
That should pretty much be it.
Naturally there is always a change that some existing other NAT configuration overrides this NAT configuration and traffic will therefore never get matched against this Static PAT configuration
Naturally you can confirm which NAT rule is hit with the "packet-tracer" command.
For example considering my above Static PAT configuration I could try this
packet-tracer input outside tcp 126.96.36.199 12345 188.8.131.52 3389
Hope this helps
Remember to mark the reply as the correct answer if it answered your question. Or ask more
Also I made a NAT 8.3+ document earlier here on the forums if you want to take a look. Though at the moment it only gives examples of some very basic NAT setups