Workspaces should almost always be thought of as your company. This is what you will invite team members into and what you will upgrade, should you choose to do so.
After I've upgraded, are all of my workspaces on the Plus plan?
No. When you opt to upgrade, you are upgrading that workspace and not your account. This is to keep the seat numbers accurate (i.e. You cannot upgrade a workspace for 5 users and then create an infinite amount of workspaces that can hold 5 users).
Can I have a personal workspace and a company workspace?
There's no limit to how many workspaces you can create, though we'd recommend keeping all of your data in one workspace This is because workspaces don't share information so you wouldn't have an accurate overview of each day, nor would the reports take into account the entries allocated in another workspace.