The main reason to convert a bathtub to a walk-in shower is to make it easier for someone who has limited mobility to take a shower. If you or someone in your household is finding it difficult to step over the side of the bathtub, several options could make the bathroom much safer and more convenient.
The biggest issue is the location of your drain. You have two options: move the drain, or find a shower that has a drain in the same location as the tub's drain. Typically, bathtubs have a drain at one end, under the faucet, and shower stalls have a drain in the center. However, some plumbing-supply houses carry shower bases with unusual shapes and drains in various locations. After you install the base, you will need to find a shower surround or put tile on the walls. Depending on the size of base you choose, you may have room for a tiled bench seat or a linen closet next to the shower. You may or may not need to move the shower head, but you will probably have to move the faucet control and take out the tub faucet. The drain will be converted to one that is always open, rather than one that closes to fill the tub, but this is easy to do.
Instead of using a premade shower stall or pre-built components, you could design your own shower using tile. To do this, you build a shower pan to keep it from leaking, and tile over it. You may be able to do this leaving the drain in its current location, or you may want to move it to the center. Your shower will have a lip to keep water from draining onto the bathroom floor, but it will be much easier to step into than a bathtub. This option gives you the most flexibility in choosing the look of your new shower, but it is probably the most expensive and the most time consuming.
There are professionals who will come and cut a notch in the side of your bathtub and refit it with waterproof material that is attractive and coordinates with your decor. This will give you the ability to step into the bathtub to take a shower. The resulting configuration has a short lip on it to keep water off the floor but has a wide opening that requires only a short step over, rather than lifting your legs over the high edge of a bathtub.
Handrails can also be installed to make it easier to enter the shower. This is the least expensive option of all and also causes the least disruption to the home, especially if the walls around your bathtub are already designed so the tub can be used as a shower.
If your situation requires a shower that is accessible by wheelchair or walker, where there is absolutely no lip at all at the edge of the shower, your best option is to re-do the floor of the entire bathroom, installing a drain in the floor in the shower area. This would be similar to the tile shower option, except you would make a much larger custom shower pan that extended under the entire floor, and the floor would have to slope toward the drain. This is obviously an extreme option, but would be well worth it if it makes showering easier for someone with limited mobility.