If your version is older than 9.0, then no chance to open project created by windows version on Mac, as the project files (.wve & .wsve) and software are not interchangeable between the two operating systems because of the software architecture. As alternative, you can export the project to a video file first, such as .mp4 from windows version first. Then import the video to the project created by Filmora for Mac as input.
And for project made with 9.0, you can archive the project first then open in different operating systems.
It is not suggested to open project made with the old version to new version9, as they share the different software architecture and built-in effects. So you are suggested to finish the project in older version and export it first. Then create new project with version9.
Yes, you can adjust the aspect ratio if you are using Filmora9. You just need to select "project settings" under "file" to adjust it directly.
Yes, upgrading to Filmora9 is free. You can use the same Wondershare ID account to activate Filmora9.