This question should have probably been answered before your baby was born. If your crib doesn't convert and your child is ready to move up, you can't do much but cry over spilt milk (no pun intended) and buy a brand new bed.
If, however, you're one of those parents who like to plan ahead and you're furnishing a nursery for your newborn, do get a convertible crib, which is a crib that converts to a toddler bed by taking off the front rails.
The benefit to doing it this way is that your child will already be familiar with the set up. It's the same bed they've always slept in with the same comfy mattress. The transition may be easier for everyone because the bed doesn't have to move, the configuration of the bedroom doesn't have to change, and your child will be able to make the change to a bigger bed more gradually.
This is by far the most affordable solution, too, because you don't need new rails or a new mattress. The only thing you may need is an adjustable rail to keep them from falling out.