Swopping out old 737-300´s

the 737-700 is faster but the 733 has a shorter turn time, so if you have really short flights on your 733s it's possible the 737-700 isn't fast enough to do the trick

i would recommend using a 737-800 in place of a 737-700, it has the same turnaround time as the 737-700, but seats 40 more, so you can increase your revenue

As already stated, there is an issue here with difference in speeds and turn times between these aircraft. If your schedule is full, you may find that there is not enough time to swap things over.
Try moving one or two flights at a time and see if that works.

If it doesn't, reply here.

when i replaced my 737-300s with 737-800s i just cut one flight and that was enough to allow me to replace them, i ordered more -800s than the -300s i had and moved the unmoved flights (from pretty much the same airport) over the one of the unused -800s

you could do the same with a -700, again i recommend an -800 since it seats more than a -700, the -700 seats the same amount as a -300 but costs more to operate, so your profits will fall, -800 is more economical.