It depends on if the VA wants to do it again, if it met the right amount last time even if it wasn't hugely popular and is feasible.
With the Yamaha group, they are not a unified group. Their third parties who used Yamaha for release. So unlike the Internet co, ah software or Crypton Vocaloids, its random on whether or not they'll ever see an updates.
From what I've seen, all V3s can run in V4, even if they are limited trials without a key. Hell, I'm sure if they wanted to those who earned Galaco's prize vb or got Ring's βvb could install them on a new computer which has V4 and run them for a couple weeks untill they expire. I think that V4 has it built in so it can read V3 (and imported V2) voicebanks with ease. In fact, with this feature I don't really see a need for many V3s to be updated. Only those who I feel that GWL could be usefull (like v flower) or are hare to come by and a V4 release could make them easier to access (like Rana. For example, the updates for both Zunko and Rion I feel weren't really necessary however, if Lily or MEIKO were updated the GWL would be beneficial and complement their voices. Also, if they re-recorded Lily I think it may add some extra clarity that her V3 lacks. The only reason that I found Yukari and GUMI's V4 updates to be beneficial was because they got some extra, brand new voicebanks. As for Miku, although the GWL did work for her (and there's that E.V.E.C. thing), I feel that her V4 update was mainly a cash gab and the only VB of hers that actually needed the update was her English which from what I heard is rather lacking.
I wasn't expecting this post to be nearly as long as it was
I'm personally more on the side where I hope (even if it is unlikely) VOCALOIDs with languages other than Japanese would be updated. Literally the only other language doing that is Chinese since Tianyi V4 is in the works. Even though I love the Chinese VOCALOIDs so much and even though I believe Tianyi (and the other Chinese V3s) deserve updates, I just find it disappointing that the other languages aren't likely to be updated. Those VOCALOIDs could totally use the new perks and improved qualities that the V4s have.
And unfortunately part of the reason they're not being updated is because they didn't sell well, they're not popular enough, or it's simply too expensive to update a VOCALOID. There also may be fears that if they bothered to update a VOCALOID, they risk it being unpopular twice and wonder if it was even worth it in the first place. :/ If I recall correctly, VoctroLabs is even out of the VOCALOID loop. Even if Maika was their most popular VOCALOID, she, Bruno, and Clara still aren't doing well for Spanish VOCALOIDs overall (from what I've heard, I could be wrong here).
As for YAMAHA, I usually tend to think YAMAHA is a little unpredictable at times. I mean, they just throw their VOCALOIDs out the window when releasing them. I don't know, maybe Mew V4 can be a thing if YAMAHA and Miu Sakamoto are up to it. Or maybe they'll literally just throw growl on her and call it a day, who knows? I mean, like, 2 years into V4 and we still don't have VY2v4 and yet we have all these other new characters and updates lol.
Tbh, if v3 vocaloids are soft-voicetypes, I don't see anything benefits upgrading them to V4. GROWL function doesn't mean anything to a soft voice. I love MEW a lot and she's one of my most favorite vocaloids, but I see no point in updating her, unless they give her another append. The only reason I can possibly think of why Tone Rion was updated is because they want to market Yumemi Nemu as a new vocaloid, and want to show evidences that the VP has been involved in the community before so it raises more sale for Rion, yet complements Nemu as a new member and simultaneously, more chances of customers buying them as bundle. If they have not introduced Nemu, I don't even think Rion stands a chance to survive
Internet co holds their license so they aren't Yamaha vocaloids... It makes things easy and difficult at the same time as we know Internet co does update their vocals. However, they don't always rush to update... Ryuto is an example as he originally wasn't to get a update but did only for Gachapin's anniversary 40th.
with professional vocalists though, getting them into the recording studio is difficult so you may get just a straight up "add gwl" moment like Gackpoid V4.