In the following page there are a number of modification needed to convert a desktop application to mobile application.
For sure, it is better to start from scratch a different platform, but sometimes resources are limited.
FixInsight could have options to show modifications needed for this transition, some of them are very common, as shown in the following page and do not need complete rewrite of existing codebase.
http://docwiki.embarcadero.com/RADStudio/Sydney/en/Migrating_Delphi_Code_to_Mobile_from_Desktop