Why don't you contact the GM of the hotel, wishing him well with the changes, reminding him of the promised benefits; explaining that you would like to maintain your loyalty to the property and ask him how he will respect the promises already made in the light of the change in affiliation.
He may offer the same or some alternative, might even better it as they may be contemplating a drop of business due to the change and may want to particularly target loyalty retention. But knowing means no suprises and offers you the chance to plan and I see no downside with asking.
Already contacted the Guest Service Supervisor (do not have the name of the GM) yesterday as she has has been very nice during my last stays. Waiting for an answer now...
Had a similar situation a few years ago with the former IC The George in Edinburgh. Had booked a room on an advance purchase rate while it still was an IC - a few weeks later they changed the owner. But they honored all Ambassador benefits on my stay.