Even some of the battle-forged Business Applications MVPs are not immune and descent into sheer panic when their forms go missing after the upgrade.
Dynamics 365 upgrades do not delete any forms and while some things may not go as planned, nuking your hard earned customizations is not on the todo list of the upgrade process.
Look under Inactive forms list – bet you that your beloved missing form is in there. It is possible to have two forms using the same name and the upgrade process will deactivate some of them that have the duplicate or conflicting names. Unfortunately, the process can be quite indiscriminate, leaving the useless now mobile express form in place while deactivating your hand-crafted Information jewel.
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Just stop naming your forms Information, will ya?