Let's face it. The Lib Dems have really screwed up on tuition fees. It's possibly our most public screw up ever. Our candidates made high profile personal pledges that they should have foreseen not being able to keep. The party is now in the position where breaking these pledges is the best option, for the following reasons:
unexpectedly fair. They are progressive (which we didn't expect before the review was published), they are workable (unlike a graduate tax, as Labour are soon to realise) and they will improve the quality of teaching (unlike free tuition) and will deliver improved access for students from poor backgrounds.* Indeed the policy beats the Lib Dem manifesto pledge on all of these tests. When a better policy comes along, we should back it, even if it costs us popularity - otherwise we're doing the wrong thing simply out of self interest.
2. Further to the above point, if we keep up the pretence that our original policy was fairer, we will be fuelling the misconception that University is inaccessible to the poor. This misconception must be tackled, because if potential students incorrectly perceive they can't afford to go, they won't, and social mobility will be entrenched. It is our duty to promote at every opportunity how the Coalition's policy will improve access to Higher Education. If you back the wrong policy, you inadvertently back everything you've stood against.
3. Even if you disagree with the analysis above, we realistically can't stop tuition fees from going up. To do so would mean all our Ministers voting against the government, thus ending the Coalition, and bringing on a Tory minority government or a general election. We can't let a single issue jeopardise the 91 other Lib Dem policies that can be achieved as part of the Coalition.
Backing the Coalition's policy is the right thing to do, but means breaking a promise, and when you break a promise, you apologise. It will hurt, it may cost us support, but it's right.
* I encourage you to read Ewan Hoyle's excellent blogpost for more on this.