When you change the printer setting from a Form 1+ to a Form 2, Preform forces you to regenerate the support structure.
In old versions, if you wanted to maintain the existing lattice you could bypass this prompt by changing your settings to the new printer first, and then opening the form file containing the model. It would also work when you want to change resins.
That trick no longer works in the latest version of Preform. (I’m guessing it got 'nixed around the same time as they implemented the awesome ability to import old contact points and sizes)
Is there another way around this? Can I manually edit the file to trick Preform into thinking it was saved with a different printer?
I know most often it’s appropriate to regenerate the support structure. But there are times when I keep it. e.g. At the moment I’m troubleshooting a print that’s failing on my Form 1+, and I want to try out an identical print on my Form 2 without changing any other variables.
If there’s no workaround, it might be nice if a third option was added to this dialog like so:
Thanks!
With the last version of preform I’m getting this prompt when changing the resin on a .form file with supports already in place. By pressing import it keep the supports.
EDIT : my bad, I read diagonally and misse some stuff. My post is obviously irrelevant to your issue 
Yes, we added the ability to reuse supports when switching resin types, but not when you change the printer type from a Form 1+ to a Form 2. Although the supports for the two printers look similar, there are actually some important differences. Because of the differences in the way the two printers perform the peel, I don’t think we could really qualify Form 1+ supports on a Form 2, or vice versa. Sorry about that.
And yes, there used to be a bug where importing a FORM file that was created with a different printer type or resin would quietly load the file without telling you that it was possibly setting you up for trouble. The ability to reuse the support locations when changing resin types was actually part of the fix for that bug, because we knew lots of users needed this ability. But doing the same for printer types seemed too risky.
Yep, I knew that bug well and relied on it to work around my corner cases. I used to work with a programmer who was known for the saying “That’s not a bug, it’s a feature” and to me this one really was 
I do understand why you don’t want to open this up in the UI. Could you help a guy out and give me a hint where I might make a hex-edit to the .form file to trick Preform into thinking it was saved with a different printer/material?
EDIT: Actually, I skinned the cat a different way. Temporarily reverted to an old version of Preform (2.13.1) to change the settings, then switched back to open the new file.
Now that’s neither a bug or a feature, but a hack
No, as I said, we implemented the “Import supports unchanged” option before fixing this bug because we knew folks depended on it.
It is important to realize that in some cases this can get you into trouble because different materials may need different support structures. For example, importing supports generated for a GP resin when switching to Flexible is probably a bad idea. But it did seem like a useful feature because something like switching from Grey to White is generally safe and useful.
On the other hand, we didn’t implement something similar for switching printer types because that’s almost certainly going to get you into trouble. The details of the support generation is tuned to the printer type and trying to use supports generated for a different printer type really really likely to get you into trouble.