Upgrading a Sport license to a Private license - is a new student license necessary to solo non-LSA aircraft? I hold a sport pilot certificate and a valid third class medical. I'm pursuing a private pilot certificate and receiving training in C172's and C182's, though I also have access to a C162. I was wondering if I can be signed off to solo in the non-LSA aircraft without going through the process of obtaining a student pilot certificate, and just getting the logbook endorsement as follows:


  I certify that (First name, MI, Last name) has received the required presolo training in a (make and model aircraft). I have determined he/she has demonstrated the proficiency of FAR 61.87(d) and is proficient to make solo flights in (make and model aircraft). 
  4/25/14 J. J. Jones 987654321CFI Exp. 12/31/05


I have several solo missions that could benefit from additional power and useful load provided by the larger aircraft, but with the new student pilot certification process, I'm wondering if it's worth the hassle.
You do not need to get another student pilot certificate.  The endorsement you need is not for student pilots.

70. To act as PIC of an aircraft in solo operations when the pilot does not hold an appropriate category/class rating: § 61.31(d)(2).
I certify that (First name, MI, Last name) has received the training as required by § 61.31(d)(2) to serve as a PIC in a (specific category and class of aircraft). I have determined that he/she is prepared to serve as PIC in that (make and model) aircraft. Limitations: (optional).
/s/ [date] J. J. Jones 987654321CFI Exp. 12-31-19

The answer lies in Advisory Circular 61-65.  Version F is the latest.