This is a guest room. Your guests are more likely to take a shower than a bath. If you sell this house in the future, this guest bath is more likely to become a child's room where a shallow tub makes perfect sense. This bath looks great and works fine. Turn your attention to something else.

Cast iron tub is likely much better quality than a new acrylic one; you will not see much return on ripping out a functional tub to replace it with something taller, so unless this is going to improve your enjoyment of the space, I wouldn't bother. Your guests aren't going to look at the current tub and think, "I wish this tub was taller." In fact, as mindshift pointed out, if your guest needs to bathe a child, they will appreciate the tub not being so tall. (I have heard from parents of small children that a standard tub is much easier than a deep/soaker tub, as they have to pretty much climb in with the kid in the deeper tub.) If you have shorter guests or those who have trouble getting over impediments, they will also appreciate the lower step into the tub. The deeper tub will be appreciated by those who are taller and need more water depth to get a good soak (and won't care as much about stepping over a taller side), but given that they're a guest, they can probably make due with a shorter tub for a couple days. The parent can also manage with the taller tub for a bit, as well. Someone who doesn't have the coordination to get over a tall side may have more of a problem with the tall side, unless there's another tub or shower they can use.

Assess what your guests need, and/or if this would be something you would get use from. Maybe you have someone who stays that you know would really appreciate a deep tub, but personally, I would leave it. If it's just for aesthetics, I would definitely leave it as is.

Thank you Mindshift & BlueberryBundtcake for your perspectives! We have 7 little grandkids to bathe on occasion & will surely appreciate lower sides then.