Can I license a film score and still have the rights to sell the film to distributors without a new contract with the composer down the line?

You need to have this contract worked out and written by a good music and film lawyer. When I write such a contract, I figure out what the parties are agreeing to and then write the contract in clear plain language to reflect that agreement. It sounds like the lawyer writing your contract will need to consider all angles and possibilities and work out how you and the composer agree to treat each situation. Some of the situations you will address will be film distribution, possible sound track distribution, use of the songs for other purposes, such as individual sales, mechanicals, performing rights, copyright ownership, derivative works, and on and on. On each of these, the lawyer will work to determine how it is you two agree to handle it, who will have the right to make decisions, who will handle any money, how that money will be divided for years to come, even as elements of the project change hands. Then the lawyer will write it all up in a contract that makes all these things clear so they can be understood by both of you and by everyone working for either of you for years to come, as well as by any judge or arbitrator. Working on such a contract may sound complex, and it is, but to a lawyer such as me, it is joyous work that I love doing.