In Glain's newest hack, titled 'Unit Bench Hack (10 extra unit slots!)' you have the ability to bench up to 10 of your units to create space for new units. Units can also be swapped out with Benched units, at the cost of the currently earned, but unspent JP. Of course you should not be expecting to be able to bench Ramza or Guest units, but you will also not be able to bench units who are set to Missing, currently on a proposition, or with ??? stats. Rule of thumb seems to be, if benching the unit would break the game, then you cannot bench that unit.
Why do we have to give up Unspent JP you may ask? Well, for space of course. Like everything awesome that is done here, space is always a concern. I am definitely not an ASMer, and won't pretend to know how this all works. However unspent JP was the easiest, or at least the least intrusive thing to get rid of while still retaining the remaining data and having enough space to bench 10 units, the alternative was keeping unspent JP and reducing the bench-able units to 6.
If you don't understand what unspent JP is, think of it like this. You had 154 JP, you used 150 of it on an ability, and have 4 left over that you cannot spend since nothing costs 4 JP. Those 4 JP are lost when benching a unit, so make sure to spend any JP you are able to before benching your unit!
All in all, this hack is a HUGE improvement to the current roster limitations. I cannot tell you how many times when I used to play Vanilla, that I wished I had more space for generics to do propositions, but that I could still keep some of my unique units on the back-burner. Glain has more than accomplished this! To go along with this hack, he has also released another new beta of FFTP .493, so be sure to check it out and show him some thanks!
You can find the 'Unit Bench Hack (10 extra unit slots!)' packed with the newest FFTP HERE, or you can download the updated Glain.xml HERE