I moved my investments (& cash, in USD) from my Vanguard (US) brokerage to Questrade in Canada. In hind sight I wouldn't move cash through a brokerage transfer as it took months as they actually mailed a cheque, assets were transferred within a couple of weeks. I don't think you can trade in your US investment accounts if you don't have any tax status in the US (I might be wrong). I still have the US accounts but don't trade through those nor do they have any assets. I've setup a RBC cross border account for moving money between US & Canada. It is very convenient for paying US credit cards and such bills. They do charge an annual fee of $40 (Canadian banks are notorious with fees). After trying various ways of converting USD<->CAD norberts gambit as mentioned by
@sk987 is very cost effective although it takes a few days to journal over the assets so you might get a different rate. If you're planning on active trading, I would highly recommend Interactive Brokers, their trade commissions are the lowest (especially for options) and you can even do forex transactions at market rates (& negligible commissions) without having to do norberts gambit.