Since I hate searching for the fix myself when I forget what it was, here it is in all the glory:
1) Open the project folder and locate the .csproj file
2) Edit this in notepad and locate the FileUpgradeFlag tag - this will probably look something like this (note that the number may be different but 40 seems to be common):
3) Simply remove the number here so that the line looks like:
4) Save the file and close notepad (or whatever editor you used)
5) Open the project again by using Visual Studio 'Run as administrator' and opening up the project using the Solution (.sln) file
Viola, project opens normally and does not prompt you to convert!