The licensing scheme on MX5->MX10->MX40->MX80 was introduced back in 2013/2014 so nothing new 🙂
You have the option to upgrade an MX5 to an MX10 allowing for usage of the second MIC slot. In this slot you can install a two port 10G XFP MIC (MIC-3D-2XGE-XFP).
Only way to get the onboard XFP ports functional is to upgrade to either MX40 or MX80 with 2 or 4 active ports.
There is no option to only upgrade with one XFP port.
You will get a license upgrade key changing the behaviour of the chassis which allows activation of the onboard ports or second MIC slot.
Initially the first shipments of MX5/MX10/MX40 were actually just MX80 chassis without builtin limitations... this doesn't mean you were allowed to use them anyway. If you originally received a MX80 chassis this will not change during software upgrades.
A last comment; If you down own the MX5 today, I will suggest that you look at the the MX104-MX5 bundle which is a MX104 including one MIC for the same price as the original MX5.
MX104-40G-AC-BNDL is also an option where two of the onboard SFP+ ports are active and two MIC slots are available for further expansion.
I hope this clarifies a bit, let me know if you need more information 🙂