From Ydun's Medium Wave Info:
RTBF 621 kHz - Since this morning (17 December) 0459z, the following message is broadcast at the end of each hour on 621 kHz:
"À compter du 31 décembre, La Première va cesser d'émettre en ondes moyennes. Pour continuer à nous écouter, basculez votre récepteur radio en FM sur la fréquence de votre ville. Vous pouvez également nous écouter en direct en DAB+, via l'application radio player, ou sur auvio."
"From December the 31st, La Première will cease transmitting on the medium wave. In order to continue listening to us, switch your radio reciever to FM on your city's frequency. You can also listen to us live on DAB+, through radio player app, or auvio."
The message seems to be only broadcast on MW, as it is followed by the end of the advertisement. Curiously, the second sentence is (almost) the same as in Allouis's equivalent announcement in December 2016.
I did not hear any such message on VivaCité (1125 kHz); neither did I find any information about the time of the last transmission, so I can only suppose that La Première will become silent at 1905z (20:5 local) as usual.
RTBF's website mentions the possibility to listen to RTBF international through Atlantic Bird 3 satellite:
73 Vincent F4HPZ (17/12-2018) via MW Circle