Currently, import-only formats are converted to one of the editable formats (MOD, S3M, XM, IT or MPTM) in OpenMPT. This can cause many files to not be played correctly compared to libopenmpt. This limitation exists for at least two reasons:

• Editing code only assumes editable formats in many places (e.g. checks if a UI feature should be enabled or not)
• Discrepancies are to be expected when re-saving the file in an editable format, leading to nasty surprises if the user doesn't verify the file instantly.

How can we work around this?

• Keep editing disabled while the file is in its original format. Not very usable.
• Don't change MODTYPE of imported file until it is saved. Display a warning that the file might sound different now.

Things to keep in mind:

• UI code should probably use CModDoc::GetModType instead of CSoundFIle::GetType() (make the latter private to prevent misuse in UI?), and CModDoc::GetModType should forward the result of CSoundFile::GetBestSaveFormat instead (careful: the return value of this may change based on editing in some formats)
• How is autosave affected by this?
• Some loaders use #ifdef MODPLUG_TRACKER and convert effects / envelopes / etc. differently for OpenMPT / libopenmpt. This would instead have to be done in ModConvert.cpp.
• ...