It doesn't usually boot them from their seat. But it will make it much harder to move seats or to fix them if there's an equipment or schedule change.

EUA looks at the LOWEST status of all passengers in the PNR. If the lowest-status person is a base member, nobody on that PNR will be upgraded. If the lowest status person is elite, EUA will only process upgrades if it can find two "R" seats.

The only exception is a Plat traveling with exactly one companion of any status (more than one companion = the rule above applies). In that case, both will be eligible for EUA at the plat window and all opportunities thereafter (although EUA will still only process their upgrades if there are two or more R seats available).