but I was thinking get notification about two weeks or so before the looming deadline and being able to pay the upgrade (eg, silver to gold) at leisure in advance so it is seamless at changeover time rather than an abrupt hurdle
Yes. So this is possible now, from July 1, now that Stripe is the only gateway.
If you are on Blue, silver and gold are always available to purchase at anytime. Likewise if on silver, gold is always available.
Trigger notices come in at 100 & 50 posts prior to the level-up requirement.