Assuming that you have not been taken back to work making at least 85% of your pre-injury earnings, or have not gone to work elsewhere, making at least 100% of what you made at the time you were injured, they are required to begin making reasonable advances of permanent disability payments starting two weeks after your temporary disability payments end. If they haven't been making permanent disability payments up to now, then they were required to bring them up to date in order to avoid exposing themselves to penalties for unreasonable delay in providing benefits. Beyond that, getting money today is pretty much always preferable to having to wait to get it.
You are not waiving any rights by cashing or depositing the check. You are only bound to something if you sign settlement papers that are then approved by a judge. Nor does taking the money prevent you from asking to have the rating reconsidered. Also, this money has nothing to do with your right to the retraining voucher.
Whether you are entitled to the 15% increase in benefits depends on a few factors, including whether your employer had at least 50 employees at the time you were injured. It also only applies to injuries between 1/1/2006 and 12/31/2012.
If you and the defendant in your case are both agreeable to working out a settlement that will pay you extra money to cash out your right to future medical treatment, and settle everything for a lump sum of money (what we call a "compromise & release"), you are still free to do so.
Before you make any more decisions, however, I would urge you to have a free consultation with an experienced worker's comp attorney, and preferably a certified specialist. You can find attorneys on this site and at CAAA.org, the organization for attorneys who represent injured workers in California.
The Insurance was probably paying accrued benefits. You are supposed to receive benefits consistently until exhausted, so a gap in past payments may have been recently corrected. The SJDB Voucher and benefit increase should still apply if you are not returned to work.
We offer general concepts, but you should give ALL your facts to a licensed Attorney in your state before you RELY upon any legal advice.
In addition to the detailed answer given by Mr. Batchelder, I would point out that you do not get the voucher that can be used for retraining until after there is a final approval of a settlement by a judge. Thus, if you resolve your case by way of a stipulations with request for award, the carrier has sixty days after the judge okays the Award to send the voucher.
If you decide to close out your case by a Compromise and Release, in which you sell your right to future medical care and your right to reopen, be very careful about settling your right to the voucher. Check the paper work carefully to make sure you don't give it up.
As recommended, a free consultation with a workers' compensation attorney would be a good idea.