3 Could we change the time?
Could we change the time? I'd like to go over some issues before the meeting. Do you think you can come an hour early?
Could we have the meeting at 1 o'clock instead of 2?
4 Let me know when is convenient for you.
I think it is better if we could discuss it in person rather than via e-mail. Let me know when is convenient for you and we'll schedule a meeting.