have to vs.must - "has to" vs "must" in passive

I've been recently writing a technical document and I wrote something like
It has to be possible to come back to the old version. (paraphrasing here, but I think sense is conveyed)

However MS Word suggested change to
It must be possible to come back to the old version.

I've been looking through the Web, but couldn't find satisfying explanation of this suggestion. I think I understood the difference between I have to... (there is someone external forcing me) vs I must (meaning I have an internal need) but I don't understand how to apply it here.

Could somebody care to explain?
Thanks!
Kris

Answer

• It must/has to be possible to come back to the old version.

As noted by xbnm, this isn't about "passive" at all. Forget passive.
And it isn't about why Word suggested one version or another. Forget Word.

This is about the fact that, while must and have to mostly mean the same, they're ambiguous.
All modals have two senses: one (called deontic) that has to do with obligation and permission,
and another (called epistemic) that has to do with logic and probability.

So, does the sentence above something appear on a specification for a software update protocol?
That would be deontic, and it would impose an obligation on the design of the software.
Or is it a proposition that any sane user would expect any sane designer to include?
That would be epistemic.

Interestingly, must seems to be preferable in the deontic sense, because it is more formal and therefore threateningly legal-sounding. In the epistemic sense, however, there is no difference in meaning between must and have to.