Switching Membership Terms

can i switch membership lengths after a month? i want at least a six month membership and see it is cheaper that way than monthly. however, i see you must pay at once. i want to start soon and will not have the whole six month cost. so can you pay for one month membership, then a month later switch to a six month and pay for a six month membership then? by the way, capitals and some punctuation arent working in this email.