I currently have PureFibre 150, because that's all that was available when I first signed up. Now I see I can go all the way up to Gigabit. Not sure which plan I will choose, but I am wondering if I do the upgrade on the MyTelus website, will the change be instant, or relatively quick (few hours?), or will I need to wait for someone to show up, or new equipment or something?
It is not instant.
You basically are just generating a trouble ticket in their customer service system for a sales rep to go and process the upgrade order for your request. The upgrade orders are still done by a human so you have to wait for them to process the request. For the time it takes, if they are busy it could be a few days. Typical would be the next day. If they are not very busy it might get entered the same day you did it.
If it's a speed bump and equipment on site is compatible there will not be a tech visit. They just enter the order, and the provisioning system will be reprogrammed with your new speed.
If you were upgrading from one fibre plan to another they wouldn't need to visit unless something like an old version of a router they plan to stop supporting needs to be swapped for newer ones.