I'm going to leave my money saving experience here:
I'm in the US and wanted to go from K11 to K13U. Buying through NI, this was coming out to ~$330 after tax.
I went to Best Service, and got the upgrade for $315 on PayPal, and then, as stated on their site, emailed support and said that I didn't want the box shipped. They sent me a refund of about $30 on shipping. I saw/used the serial from my user section on the site within minutes. Can't say whether or not they'll continue to have serials immediately available though.
Result- would have cost $330 direct from NI, payed $285 total from Best Service, no extra waiting.