It's hard to see any renegotiations having only an impact on immigration into this country, rather than out. And indeed in general it's difficult to see the rest of the EU accepting changes that benefit our national interest whilst simultaneously hurting the interests of the EU. Cameron's attempt to negotiate a better position is not likely to achieve anything more than putting off a discussion that we should probably be having around about now, or at least not that long after the next election, and should be framed as a vote on continued EU membership rather than on what piecemeal changes he does manage to get out of the negotiations.
In the meantime rising support for UKIP might make it even harder to leave the EU, ironically -- although that might not be true if the votes are coming even more from Labour/ LibDems than from disgruntled Tories.
One way or another, we should try to settle the issue as to whether or not we want to stay in the EU soon.