'We better change the subject' is not grammatically acceptable UK English, though the expression 'had better' is in common use as in 'You had better put your coat on' (it's cold outside ), or as above, 'We had better change the subject' ( Before someone gets hurt?) Druckles' suggestions of 'we should' or 'we ought to' are good alternatives if the translator is prepared to lose the 'besser'.
No, that would not work. In this sentence, "We had better..." means that it would be advisable, or recommended, to change the topic/subject. It isn't about making the topic/subject itself better, or about making anything better. Plus, "change" can't substitute for "make" in the way that you are envisioning. Good question though.
For this to be translated into "We'd better change the subject", wouldn't one of these be better: < Wir sollen besser das Thema wechseln. > < Wechseln wir besser das Thema! > To me, the original sentence would translate (very awkwardly) to < We are changing the subject better > or maybe < We are improving the subject > (I know that "verbessern" is more literal for "improving")
If one restructures the sentence to: "Better (that) we change the topic" the German verb makes more sense to my English sensibilities. It takes out the "had", and puts it clearly into the present tense. "had better" has an element of threat to it, and suggests that "sollen" might be used here. Once again, we English speakers fall prey to the German propensity to mix up sentence order!
As my knowledge goes : We'd implies "We (woul)d or (shoul)d* or (ha)d".
And in this case we're dealing with "we'd" = "we had".
*I'm personally not aware of "we should" being contracted to "we'd", but a quick Google search says so, so who am I to disagree? :P
So this is a form of a subjunctive.
Thus, could this be translated as "Wir würden lieber/besser das Thema wechseln" or "Wir hätten lieber/besser das Thema wechseln". ?
With "lieber" the first sentence means "We would prefer to change the subject.", which isn't quite the same as "We'd better change the subject."; and the remaining suggestions aren't grammatically viable.
Had better is an interesting grammatical phenomenon, that—AFAIK—has no direct equivalent in German. I'm more taking a stab here than quoting a grammar textbook, but my gut tells me that "had (better)" is a subjunctive form of "to have to" equivalent to the modal verb "must".
This would give us two verbs to choose from when translating to German: müssen & haben (zu). Obviously, we're still dealing with the subjunctive here, so we'll need the Konjunktiv verb forms (KII to be specific):
Wir müssten das Thema wechseln.
Wir hätten das Thema zu wechseln.
Now, do I think the two above sentences are suitable alternatives to the suggested translation?
But I think that's as close as we can get trying to somewhat match up the underlying grammar behind the "had better" construction in both languages.