You mention you "had silver status when [you] booked" (presumably in March 24?). What is your status now? If your status dropped, then there is always a risk that advanced seat reservations will drop before flight time, sometimes at the most inconvenient of times. It often happens during one of the frequent configuration changes a flight can experience and there is always a risk that at the time the seat assignments no longer supported by the required status there will be no decent alternative to book, even for a fee so that is the big risk.

Others' experience shows that it is really a toss whether seat assignments survive when people drop status in general, but if there is any configuration change at any point, they are almost certain to drop. Bear in mind that if you dropped status, this could happen for your outbound too.

Having said that, no seats available to book does not in any way mean you'll be bumped. Notably, on any given short haul flights, there are several rows that will remain unassigned all the way till t-3 as BA keeps them unavailable for seat assignments in both Y and C as they do not know whether they will be used for one or the other.

Apologies if I misread between the lines and you are still silver.