Silly question: Is this going to be coming with the new head if purchased now, or are retailers still clearing out packages with the old head?
Furthermore, as someone who is new to Iron Man and Marvel hot toys, are the sculpts interchangeable? For example, could I put a mk 46 RDJ head on a mk 43 body? I prefer the no-helmet vs open helmet for an RDJ face sculpt, and a full mk 46 figure is just too expensive, so I'm considering my options.
Although I will probably just wait for a reissue of 46.
You'd have to ask the retailer what their plans are. I think some over in Asia have advertised as having done the head swap.
I don't think any US retailers would've received replacements yet if they participated. Sideshow has the product photos with the old sculpt. If you want a new head, you'd want to find a retailer that's confirmed to be getting a replacement and work it out with them. Or wait till there's reports of Sideshow starting to send out heads.
The heads are generally interchangeable though sometimes they don't fit exactly requiring a bit of museum putty or Blu-Tack. But there is a bit of a scale creep so the Mk 46 onwards are fairly noticeably larger than the Mk 42/43.
A reissue of Mk 46 almost seems inevitable but nothing is guaranteed till it happens. The reissue of Mk 47 may have under performed, and with Phase 4 finally starting to take off, they might invest resources elsewhere like making a lot of Spider variants.
At the very least, I think a Mk 46 reissue probably wouldn't happen till the 47 is closer to selling out. Considering the lead time between the announcement, production, and shipping, the hypothetical reissue might not be in-hand for many years.
Even if they announced it tomorrow, the figure probably wouldn't arrive till 2023 at the earliest.
The Mk 50 and Mk 47 are readily available, both with head sculpts if you want a figure to enjoy now.