IF transformers are less critical if you are using a
crystal filter. Many solid state rigs don’t use them
at all.
You can change the capacitor over a reasonable
range, particularly adding capacitance in parallel
to lower the frequency. (Some QRP rigs for 40m
or 80m used 10.7 MHz IF transformers that way.)
But raising the frequency may be problematic:
a 455 kHz transformer winding may have too much
self-capacitance to be used at 1500 kHz.
But traditional IF transformers used double-tuned
circuits for selectivity, and a crystal filter will
provide far sharper response than any reasonable
number of tuned circuits at 1500 kHz. So you can
use other interstage coupling methods instead.
You may still want at least one tuned stage near
the product detector to help reduce noise generated
in the IF strip, but a single parallel-tuned circuit
may well be adequate, which could be any sort
of coil, either slug-tuned, or using a trimmer cap.
A crystal-controlled converter should work well.
That makes stability less of a problem on the higher bands.
The additional loss of a crystal filter can be made up for
either at IF or AF. It may just be a matter of turning
up the AF gain a bit. Don’t worry about it until you
confirm that it is a problem.