Yes, your inital cover letter is the first element of a proposal for purposes of your eventual agreement with a new client. But you should expect to refine your initial proposal with the client as their intentions, goals, skills, etc. become more obvious during the time before you both agree to a contract on Upwork.
Always be sure you and the client are on the same page as to what major elements you will AND will not do to complete the contract. Not everything can be foreseen, but too many clients end up asking/requiring more work from their freelancers than the freelancers expected to do.
Define the final work product/deliverables as best you can before a contract is in place. On an hourly project that is not as important as on a fixed price project, but an outline of what's to be done by the freelancer is helpful for the success of any project. Many clients don't even know exactly what they need; the freelancer is doing him/herself a favor helping the client in that respect.
Good luck!
