Can I switch to a SQL on IaaS service later if required?

A direct switch from PaaS to IaaS is not possible. Though this can be easily achieved by exporting the database in to a format (a zip backup of data and schema in a single export) as offered by Azure. The file can be exported to a local storage or Azure storage (200 GB limit on Blob storage) and can be imported in to a SQL instance inside a VM or Physical SQL database server.

Refer to export procedure here