Re: No worries!
I upgraded my Lumia 735 to 10 a few months ago - rather apprehensively, it must be said. However, I was very pleasantly surprised. Generally the phone feels snappier, things work just fine and I've been delighted with the result as it feels like my phone has a new lease of life and should keep me going for a good while yet. There's a few things to adapt to such as the Maps app rather than the Here Drive/Here Maps/Here Transit threesome, but it's all pretty painless. YMMV tho'.