Bumping this to ask what people think the best strategy is for trying to upgrade BOS-MAD from Y to J on Iberia?

Details:
Ticket is BA-coded and Iberia-operated
Current ExpertFlyer details show the following (but we’re over 45 days out from date of travel):

IB 6166 BOS 5:20p → MAD 6:20a
J9 C9 D9 R9 I9 W9 E9 T9 Y9 B9 H9 K9 M9 L9 F0 V9 S9 Z0 N9 Q9 O9 A9

BA (IB) 4264 BOS 5:20p → MAD 6:20a
J9 C9 D9 R9 I9 W9 E9 T8 Y9 B9 H9 K9 M9 L9 V9 N9 Q9 O9 S9

Is is possible to use Avios to upgrade the reservation to Y+ and then do an AUP or POUG from there to J?

Or is a further upgrade somehow disallowed or blocked if you have upgraded an underlying paid fare with miles already?

And how does the codeshare or prime flight number issue affect all this? Does having the BA code preclude possibilities that would otherwise be available if the ticket had an IB code instead?