If you know the current bike's stack, reach and seat tube angle, comparing it to a new one is fairly simple. Stack heights often have a 20mm difference between sizes. To compare the two correctly, assume that 20mm more steering tube spacer will be used on the smaller size and subtract 6mm from its reach, then compare the two reach values. Seat tube angle only affects the seat post setback that may be required. Each degree of difference changes the seat post clamp horizontal position by about 12mm.
Stem angle can be used to change the bar height too, if the bike doesn't use a proprietary integrated bar/stem with no stem angle options. With integrated bars, getting the right stack height is more critical, to avoid a big stack of spacers.
I pay no attention to frame size numbers or letters. If the stack on the new frames is a lot taller, you may be comparing new endurance models to an older racing model. If you want lower stack, a different frame model may be needed.
Last edited by DaveSSS; 10-31-23 at 07:25 AM.