Hi I want to purchase but test and make tweaks on a testing domain name. Can I swap the licence to the main domain when im ready to go live?

Yes, you can purchase MJ for dev domain and that send a request to transfer your subscription to another domain (but it may takes some time, up to few days), or purchase it for main domain and do tests on dev domain (the only side effect should be an advertisement in MJ Basic and using of local device database in MJ Pro, in addition to notification "subscription expired" in the MJ's backend).