If you have a pump now and buy a new one to replace it, the new one is a replacement pump. Since Leslie's markets to existing pool owners, unless you are buying something you never had in the past, everything is a replacement. Its just marketing.

If you have a pump that has worked well in the past and you need a new one, it is easiest if you buy the same brand and model (but not necessarily hp). Otherwise you will probably have to do some re-plumbing to make it fit. Since most pool builders in the past have put pumps on the pool that are too large (in hp) it is always best to get a professional to examne your system. Paying a service call for this now can save much grief later.

If you are comfortable working with plumbing, electrical and water, you MIGHT be able to do this yourself. Otherwise have a pro do it, that is the way I recommend.