A couple of main changes
* it is considered dangerous to change the mode multiple times in a
script. What do we expect when it is changed after one fetch and
before a second. No, that is a timebomb.
* It makes no sense to ever change the default mode, since old scripts
will then start failing.
As a result of those two points the idea to pass in an enum value
is just a waste.
So unless someone has more enum values, the KISS principle comes
to one OP_milli-sats-mode opcode.
The multi-year approach solves the issue of sub-satoshi's getting
lost, but since a single Satoshi is still quite cheap the solution
is unneeded. We can just assume that such accidents simply pay a
tiny tiny bit of money as miner fees.
tomFlowee