I’ve done that more than 3 years ago. For this I have done:
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while editing my Lr picture I’ve been using dt for some of them. this was to learn how dt works and don’t be too “lost” when switching.
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then I’ve created a Lr → dt module (integrated in dt) to recover some data (all keywords and some edits like crop, curve,…), see Importing Lightroom Development | darktable
Don’t be too expecting of the auto-import from Lr, it is not perfect but I found that it saves me 80% of the work on each image I have needed to recover in darktable. that’s is when I need to print or export an old picture the conversion save me quite some time.
At the start I think I’ve been editing 20% slower, with the time I think I’m now editing in darktable at the same speed I’ve used to do in Lightroom as the tool has progressed a lot and some editing not possible in Lightroom are now possible in darktable.
All in all I would never ever go back to the lock-in Adobe policy.