Regarding changes in the accepted manuscript
Typically, journals will send you page proofs after your manuscript is accepted and before it's published.  A limited number of changes for clarity is usually fine at that stage (especially if it would have to be printed as errata later).

If the journal is not intending to generate page proofs (e.g. it's a rapid electronic publication format), or if there are more than a handful of changes (six or so), or if they are large enough so they might change the layout of the whole paper, simply contact the editor as soon as possible explaining exactly what you wish to change and why.  Assuming all the changes are minor and make the paper better, the answer is usually that it's fine.