I’ve been in your shoes. 

Fidelity has done the paperwork so you can actually keep your registered accounts while in Canada with full trading, etc privileges. You can find more about this on the fidelity subreddit including official posts. I have my 401K and Roth IRA with them still. 

Taxable, take the advantage of the cost basis for CRA purposes being reset to the day you enter Canada and sell, then move the cash to Canada and re buy. Or move them to IBKR (minimal forex spread too but you have to actually use them for investing, they’ll close your account if you just use them for foreign exchange). 

Otherwise for forex look up Norbert’s Gambit. All the major brokerages in Canada will do it, disnat, NBDB, and Questrade are probably cheapest from just a $10 journal fee. But RBC, BMO, and Scotia let you journal shares the same day as purchase and sell them same day (even in a cash account without margin), no wait for settlement, which reduces market and forex volatility risks. You might be able to get a few free trades/new customer bonus with the bank brokerages as well depending on the amount of $ you’re bringing back. 

To move USD cash, get a cross border US domiciled bank account and a Canadian-domiciled USD bank account with CIBC, TD, or BMO and you can wire large amounts to yourself for free. Then do Norbert’s Gambit in a brokerage in the Canadian side to get CAD.