Sorry, for now, it is not possible to change a project's platform or to create a multi-platform project.
The reason why we recommend you create different projects for different platforms is because we create the appropriate type of assets for each.
Assets can be exported in the following formats:
Bitmap assets: PNG
Vector assets: PDF (iOS), SVG (Web & Android)
If you're still unsure of which format works best for you, check out this post.