So you're asking about sawing an opening into the existing tub while it remains in place? Uhhh.... that seriously seems like more trouble than pulling it out and building a shower stall in its place.

50's tubs are en vogue now, so if it's in good condition you can inquire around locally and see if someone wants it in exchange for hauling it out, or would haul it out for salvage for a fee that's less than demoing it.

For a more immediate and less expensive option, could you improve accessibility for the time being with grab bars?

I had never heard of such a thing - but here's a youtube advertising a company that retrofits an opening into an existing tub - e..g. essentially just cutting an opening into the side of the tub.

https://www.youtube.com/watch?v=t72teDgFOEE

I have no idea what the cost is or whether it actually is a viable solution but in theory it seems to be a relatively easy solution since a tub shower combo is already waterproofed for a shower. I am assuming you want to use as a shower rather than to take a bath since getting up and down in a tub presents problems.

In the meantime while you wait install grab bars - these need to be attached to studs for maximum safety because otherwise they can be ripped from the wall if you lose your balance.

You can also get one of the rotating seats which sit on the edge of the tub - you sit and then use it to rotate your feet into the tub.

Make sure that you have non-slippery surface on the bottom - my tub/shower combo was quite slippery.

You should also put in a sturdy chair with a back if you have balance issues.