Different coaches will say different things, but my general principles are to keep all your swimming as similar as possible. Regardless of whether you’re swimming short or long, easier/slower or faster/harder, the intention should be to keep your stroke roughly the same. The more that your stroke is repeatable, the better the carry over will be at any given intensity.
If you find short course a challenge in terms of distance, focus on slowing things down a little, swimming gently and generally trying to reduce your effort level. In the same way that you wouldn’t run a half marathon at the same speed you’d run a 5k so you shouldn’t be looking to swim long distance at the same speed you would swim 750/1,500m.
For a longer swim, you may stretch out a little longer forward and back, whereas in a shorter/harder swim you may well focus on a slightly shorter reach and pushing back harder to help turn your arms over quicker and increase the cadence. Other than this, your body position should remain high, strong through your core and your control of the water with your hands/forearms should stay the same. The only other slight change is that you may kick your legs less over a longer distance – this will save you valuable oxygen!