Some basics:

Any Elite of any level (FO to DM) flying on a paid ticket can designate a companion for upgrade. The companion MUST also be flying on a paid ticket, must by an SM member and CANNOT be FO, GM, PM, or DM.

If either the Elite or the Companion (or both) are on an award ticket (or pay with miles), the companion CANNOT be upgraded. If the Elite is an FO on an award ticket, there is no upgrade possible, either.

If the Elite and the Companion are on the same PNR, and on paid tickets, the Elite and the Companion will clear no sooner than 24 hours prior irrespective of the Elite's status.

If the Elite and the Companion are on separate PNRs, and the Elite has requested, by telephone, that the Companion be put on the upgrade list, then the Elite will clear at their upgrade window and the Companion no sooner than T-24.

This all assumes that there is inventory in the appropriate buckets at the time of the sweep, of course.

If the phone agent told you that Silvers no longer can upgrade a companion, then s/he is wrong IF both are on paid tickets. Otherwise, s/he is correct but it isn't new and it isn't just FO's.

But then again, I am not a Delta Agent but I did stay at a Holiday Inn Express not too long ago.