DOes this convert to a full size bed? Does it need a full size mattress when that is done?? Or do they convert to a twin bed?

Technically no. You can buy just a new mattress, but some of the damage done to mattresses ie the indentations are caused by insufficient frame support. So check your frame to make sure it's in good condition with nothing broken.

No, they are slightly different. Here is a chart of common sizes for sofa-bed mattresses:

Queen - 60" x 72" or 58" x 72" or 64" x 72"

Full - 52" x 72" or 54" x 72"

Twin - 38" x 72" or 35" x 72"

Single - 24" x 72"

A children mattress is mainly between 38 on 75 inches.

An adult single mattress is around 53 on 75 inches.

A couple king size mattress is around 76 on 80 inches.

Just the mattress should be fine since it already has a "bed frame". But since a lot of people have trouble converting cribs into day beds or full-size beds, you are probably better off getting a box spring for that. Also, find out if you have the parts that you need. There have been a lot of recalled Simplicity cribs and the parts for those recalled items are no longer available.

http://www.bestmattressreviews.com

This Model only goes from Toddler to Full Size with conversion kit.... http://www.milliondollarbaby.com/collections/emily

The Emily Mini Crib goes to Twin bed.

Did you get an answer to your question? I have the same problem.

Most if not all of the simplicity brand drop-side cribs have been recalled, including the Graco 8740.

Here is the CPSP link regarding this crib:
http://www.cpsc.gov/CPSCPUB/PREREL/prhtml09/09260list.html
http://www.cpsc.gov/cpscpub/prerel/prhtml07/07307.html

According to the CPSP information the company appears to be out of business to some degree and are not providing repair kits.

If you know where you purchased the crib, check to see what their policy is regarding recalled baby products.

We had purchased ours at walmart almost 8 years ago and were able to return it to another store in a different state for a full refund last summer. They did not need a receipt received, they just needed the manufacturer label on the part that the mattress lies on.

Because ours was older, the customer service clerk did have to work hard to find the crib in their system, but once she did, it was not problem at all.

Also, the bed is made to convert to a headboard for a full size bed and so it may be a little bit wide to work with a twin sized bed.