I'm looking to travel to the UK in November. With my United MP#, I can buy Economy Plus seats online. It requires that I enter my United MP# to get these seats. Question is, once I get to the airport, am I going to have a hard time switching to my USA Dividend Miles #? Are there any downsides here? Like are the EP seats guaranteed?

Last thing I want to do is get stuck with United miles.

Alternatively, I am considering just flying USA (will require 1 stop). I'm Gold, can I buy up into Envoy for $300 & 30K miles?