I want to formulate the liposomes in ~10 mM phosphates, but I want to run some experiments in ~100 mM phosphates. Will this cause leakage of my entrapped protein and/or loss of liposome structure?
You are changing the ionic strength of the solution. So it will have an effect. You need to counteract the ionic strength of whatever is trapped inside the liposome with the outside solution.
You could run a perfectly simulated experiment, a prototypical efforts to check! The key is the timings! What is the time duration your liposomes and its contents are safe, can be emulated (bit less) to carry forward your intended experiment. Good Luck!