Since both legs would be MFU eligible, this should be OK. The usual problem in these scenarios is a mixed WT and WT+ booking where the WT class is discounted and not eligible to MFU (even if you don't actually want/try to on that leg).
Programs: BAEC Silver; Muccis du Monde des Peluches
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That's an interesting one. I don't see why not, as in general MFUd tickets have the same flexibilility as the original ticket apart from the "upgraded part" - you lose the upgrade on a leg if you change the leg after 24 hours prior to your first leg. And obviouly if you wish to change the leg and keep the MFU, there has to be redemption availability (and also availiability in a revenue bucket corresponding to the original fare class?). But I don't see any problem with changing the W part at any time with what you propose.
I think there are others here who would know for certain.