GPS

I am leaving to live in Canada on 14th Sept.
I sold my boat during the week but I kept my Garmin FishFinder 340 and in particular my GPS unit - GARMIN GPS 450

My question is - can I load maps for the Great Lakes on my GPS and bring it with me to use over there? I will be living in Kitcherner, ONT, between Lake Ontario, Lake Erie and Lake Huron, I will be going to Maine, Massachusetts and New York on holidays with my boat over there so will still want sea maps too

Is it possible to install the USA/Canada maps and keep my UK/Ireland base maps or will I have over-ride the UK/Ireland maps?

Thanks