Switching from Mambo to Joomla

I currently have SEF Advance installed on my site which uses Mambo version 4.6.5. I'm contemplating migrating the site to Joomla since it doesn't appear Mambo is still under development. If I do so, I have 2 questions:
1) Will I need to purchase a new SEF Advance license?
2) Will my URLs change from what they currently are? This is a big concern of mine since we get a lot of traffic from Google and other sites that link to specific articles of ours.

Thanks in advance for any help you can provide.

1) If you go to Joomla 1.0 then no. If you plan to upgrade to Joomla 1.5 then you need a migration license (28 EUR).

2) Most of the major URLs should stay the same with correct settings, some may change which you can fix with redirects.

Thank you for your quick reply. At this time I think I'm going to delay moving to joomla since I don't think I'll have the time to figure out how to do the migration and updates needed with the sql database, etc. (maybe next year). I am, however, switching to a new domain. Will I have to purchase a completely new license or is there a migration/change license that can be used? I have about 600 links I need to redirect so that may take me a little while, but I have the new site pretty much ready otherwise (content moved over, etc.). I installed a trial version of SEF Advance today to make sure I had everything correct and it appears I do.

Thanks!

If you are shutting down the old domain I can transfer the license without additional costs. Send me an email when you are ready and we can arrange that.