"loose your warranty" is only a
legalese declaration, it's Microsoft lawyers telling them to say that so you won't sue them.
When I connect my Lumia 810 w/
GDR3 to Nokia Software Updater for Retail 4.1.0 (NSU) it says
In - warranty in the US in fact it only shows that Amber/GDR2 is the software version not GDR3,
so if Nokia's own NSU says that, what will whoever that sees your phone see ?