Replacing finned with "finless"

Is it ok to do this? What needs to be done when replacing with a finless? I know how to get them out. Some siding will be removed. Wouldn't it make it much easier in the future to replace the window? Why do they even use the fins in new construction? Is it that much better at being water tight or something? thanks

Nailing fins are easier to hang that way and provide superior water infiltration performance.

Even when installing a wood window with brick mould Jeld-wen recommends that you brake metal and make your own fins for sealing the unit the unit to the weather barrier. In reality they will void the warranty if it is not done.

The importance of flashing is not overstated.

Do it once. Do it right. Then you can enjoy your life.

Often the point is redundance to make sure no water infiltrates.
Yuo can bend a metal drip for the top and seal to the substrate, you can tape from on the new window to onto the sheathing down the sides and bottom.