I was a STA-21 guy and from what I recall going through that source, if you re-enlist at your regular interval (i.e. you're near the end of your obligated service, want to re-up, and haven't been applied or been accepted for a commission) but apply/get picked up for a commission later down the road, you get to keep what you received. However, if you get picked up for a commissioning program and re-enlist to meet your required obligation (i.e. 6 years for STA-21), you shouldn't get an SRB and if you do, they will take it away...all of it. There seems to be a bit of a grey area in the timing if you go STA-21 though. I don't remember exact dates, but I want to say that if you applied for STA-21 and re-enlisted after July 1 of that year (when the packages are due) and you get selected, then you won't get an SRB. There were guys all up in arms and ready to riot at the PSD brief over their SRB being taken away.
This is just what I've seen/experienced. When I got picked up I was well into my re-enlistment and I set up an extention for the remaining time. I didn't have to give any of my SRB back, even my installment for the FY I was selected. Other guys applied and then re-upped before the results came out, got selected, and found out at NSI that the SRB money they'd received had to be paid back.