You can swap an exercise without breaking the stimulus.

Match the joint action, the load position, and where in the range the movement is hardest — in that order.

Every program I write gets substituted. The rack is taken, the gym has no trap bar, your shoulder doesn’t want to go overhead today. None of that is a compromise as long as you match the right three things, in the right order: the joint action, the load position, and where in the range the movement is hardest.

The three things to match

Joint action first. A squat is knee and hip extension against an external load, so anything doing knee and hip extension is a candidate. Load position second — front-loaded, back-loaded, held at your sides — because that decides which part of you gives out first. Hardest point third: a barbell curl is hardest at ninety degrees and an incline curl is hardest near the bottom, so swapping one for the other quietly changes the exercise even though both of them bend an elbow.

Match all three and you’ve got the same exercise with a different implement. Match the first two and you’ve got a close cousin, which is fine for a block. Match only the first and you’ve got a different exercise wearing the right name.

The swaps I hand out most

Back squat → safety-bar squat, front squat, or heel-elevated goblet squat. Same knee and hip action; the front-loaded versions ask less of the lower back and more of the quads and upper back.
Conventional deadlift → trap-bar deadlift. Keeps the hinge, moves the load to your sides, drops the lower-back demand noticeably at the same weight on the bar.
Barbell bench press → dumbbell bench, floor press, or weighted push-up. The press stays horizontal, which is the part that matters.
Barbell row → chest-supported row or single-arm dumbbell row. Support the torso and a row is still a row, minus the argument with your lower back.
Overhead press → landmine press or low-incline dumbbell press, on the weeks when overhead isn’t available to your shoulder.
Romanian deadlift → 45-degree back extension or single-leg RDL. Same hamstring stretch under tension, a fraction of the spinal load.

What to change, and what to leave alone

Change the load. A front squat will land 15 to 25 percent under your back squat, and a single-arm dumbbell row doesn’t compare to a barbell row at all — find the new weight by feel in the first session and write it down. Then leave every other number alone. If the session says 4×8 @8 RPE · 120s · 30X1, the substitute is done as 4×8 @8 RPE · 120s · 30X1. The prescription is the stimulus. The implement is a detail.

Keep the swap for the rest of the block, too. A different substitution every week isn’t a program, it’s a series of first sessions, and you can’t progress a lift you never repeat. One decision, held for eight weeks, beats eight good decisions.

Three swaps that don’t work

Leg extension for squat. Same knee extension, no hip, a fraction of the load and none of the systemic cost. It’s an accessory, not a replacement.
Lat pulldown for barbell row. Vertical pull standing in for horizontal pull — different action at the shoulder, different muscles doing the work, and your upper back will notice inside a fortnight.
Kettlebell swing for Romanian deadlift. Both hinge, but the swing is ballistic and spends almost no time under tension. It trains something real. It doesn’t train that.

MINIMAL and RANGE ship with the substitutions already written in, because I assume a bench, a pair of dumbbells and nothing else. For everything else: run the three tests above and back your own answer. Getting a swap slightly wrong is recoverable in a week. Skipping the session because the rack was busy is the more expensive mistake, and it’s the one I see.