Hello Per Kristiansen ,

Yes, you need to create a new project and select the new target. The same for a Translation memory, if you created one.

Hope my answer can help you.

Per Kristiansen 

If the project is just for you and just on your machine, and if you are not using any resources that target Nynorsk specifically... and by that I mean you would get different automated translations for example... then why is it important to change the project now?  Technically, if this is just for you, you could just add your TM using AnyTM, and if your termbase is based on just Norwegian without worrying about the variants then that will be ok.  At the end of the day you just want to complete your project and save the target.  So might be worth not even worrying about it!

If you have done a lot of work on the file already and that's the reason you hesitate creating a new project then there are workarounds to getting a new project quickly set up and with all the work you have already done intact.  But I think it would make sense to know what stage you are at in terms of the translation itself.