I purchased a ski with Marker Griffon bindings, and, my boots were 10 mm less in BSL. The Griffon heel was able to be moved forward to accept my boot without a remount. I sold the skis to someone with a 10mm less BSL that me, he had to have a remount. So assuming your skis were mounted with standard Marker template spacing, you should be good to go. My Head/Tyrolia Freeflex bindings have the same downsizing ability.
I would not assume that your new boots will be 5mm less in BSL, as, boot length only changes on full sizes. The shell of your 27.5 is probably the same shell used in a 27. You will know the actual numbers only when you select the new boot.