I've had a mix of experiences. I repotted Onc. Heaven Scent while it was in spike and after repotting the spike grew longer and had more buds. So I don't think it's 100% certain you'll lose buds and/or blooms but just best to assume it's likely. I mean heck, I've gotten used to assuming that just bringing a new plant home will interfere with blooming.
Something to consider though is that if the plant is still in bloom, and if by all intents and purposes it looks healthy and the blooms beautiful the plant is doing well enough to maintain these blooms in the medium it's currently in. This is where my nerdy side loves to keep track of blooming times and what not so next time the plant blooms I can compare the size, the color, the number and duration of blooming.
I don't think immediately repotting any and all new plants just cuz is really what's best for the plant, IMHO. Consider this: the plant is already having to adjust to this new environment - unless it's not so new! - therefore repotting could potentially serve as another stresser. This would be not only due to the manhandling but also the adjustment to a new media. I'm not saying this is always the case but it's something to consider. Just as you're not guaranteed to lose blooms you're also not guaranteed to set the plant back if you leave it in this media that doesn't immediately look bad.
I've got a Zglm. Louisendorff from a show a couple months ago. It's in Promix, that makes me nervous. And actually it's two plants in one. I've got two pods on it and the blooms died a while ago so I've thought about repotting but then I find one plant has two little new growths, the other has one and they're growing fairly well. I'm at least not going to repot this until both of those little guys have their own roots. But more than anything, this plant seems to be doing well enough that I'm considering trying some promix in the future with water loving plants (have I said yet that I'm an under-waterer?).
I have been a repotting fool these last few weeks and if anything, it seems the plants perk up after new media.
I received a miltonia about 7 days from opening, repotted it (because I feared what I didn't know..what it was potted in), and its first flowers opened yesterday.
I will admit the dendrobium I repotted day before yesterday looks worse today...possibly because it jumped off the windowsill in a breeze and splatted on the floor, media flying everywhere