I have known a lot of CSA's (Customer Service Agents) who have gone on to be F/As. Some F/As do go on to become CSA's but it is usually the other way around. The jobs are similar (Customer Service and Safety) so if you have demonstrated that you perform well in the F/A role an airline won't doubt your ability to perform in the CSA role.
Southwest is different from most carriers in that they have Customer Service Agents and Operations Agents. Customer Service Agents work the ticket counter and customer service counter at the gate. The Operations Agent is the one responsible for actually boarding the flight, planning weight and balance and communicating with the flight crew. I believe the starting pay for both jobs is around $11.
My opinions, statements, etc. are my own and do not have any association with those of any employer.