It appears that it is not possible to deprecate a Gen 1 image from a plan, which has both Gen 1 and Gen 2 instance types and had already been published.
We were advised to recreate the entire plan, which is a lot of work because not only technical but also marketing and pricing content need to be reentered.
This must be an issue for vendors who have been on the Marketplace for a long time.
Please add an option to allow deprecating any image for as long as there is at least one remaining. Consequently instance types associated with the plan should update to match generations from images that remain available for the plan.
Once an image type is support within a plan, at least one image must remain of that architecture. So in this case, at least one x64 Gen1 and one x64 Gen2 image must remain within the plan. We have received feedback on this from ISVs and have an item in our backlog to address this in the future. In the meantime, you can either make a new plan, or otherwise, add a new image version with only the Gen2 image type as the latest image version becomes the default for customers to leverage on Azure Portal.
Please let me know if you have any additional questions.