So, (mostly) good news. We have recently made changes to our headend software. The headend used to base its understanding of the discs you own on the inventory uploaded by your system. That inventory is updated fairly frequently, so if you did a disc-to-digital purchase and then deleted the disc version from your system to clean things up, the headend would quickly lose track of the fact that you own the disc.
As of the recent changes, the headend preserves knowledge of your discs even if you remove them from your system. This means that going forward, you won't need to worry about having to stick the disc back in the system if an offer becomes available. This is especially valuable for customers who may be in a situation, say, where a disc-based system experiences a component failure. Since this is the sort of scenario that often prompts people to update to a download-based system, not losing the records of disc ownership greatly reduces the hassle of making the conversion to downloaded movies.
There's an important caveat here. This change applies to discs that were on your system when the headend software was updated (which was March 10 of this year). Unfortunately, it was not possible to retroactively record discs that had been removed prior to that date.