Re-enlist after signing First Termer Statement?

Yes you can, there is no negative action associated with a first term statement.

To qualify for a first termer statement you must meet three criteria: be on assignment to a location that generates an SRR that the SM must reenlist or extend to meet; never have reenlisted (extending does not count); must have less than four years active service from BASD to ETS.

If it's anything like a Dec statement you can always request to withdraw it. I'd go up the chain and also consult your S1 and Retention sections

You can always try. I didn't apply for a CJR during my first term. Back then you had to make your intentions to reenlist clear by getting a CJR (Career Job Reservation). Then they would either give you one or retrain you. I didn't get one, then about a week before my ETS date I decided I wanted to stay. I was lucky because my Commander went to bat for me and I got to reenlist in the same job with no changes. But I could have just as easily been put out the gate.