I think it's safe to just make the change. The extended form isn't in use in a VMMaker image (trunk + a lot of other stuff) so it won't affect most people. Marcel will be affected but he can update to a new VM (right Marcel)?
Here's code that checks: self sn browseAllSelect: [:m| m scanFor: [:a :b :c| a = 16rE1 and: [c = 16rE9]]]
Note that the change is unfortunate. Because the existing VM doesn't clear extA afterwards setting extA can have disastrous effects in the old VM. e.g. if followed by a block creation bytecode then that bytecode will consume the extension not cleared by the push character bytecode. But as I said it is unlikely to bite anyone. Fingers crossed.
Hacking the existing spec to use a sign extension is less harmful in the short term, but more harmful in the long term.