This is exactly my problem, since I learned classical Castilian (*) Spanish in secondary school in Ireland (fairly unusual, normally French is offered as a third language). The teachers were a mix of lay and religious (who had come back from missions in Peru), so accent-wise we were pretty screwed, as well as the emphasis being on grammar and vocabulary rather than speaking.
So now I have good grammar, a fairly huge vocabulary, but an accent that has survived almost intact after years of living in LatAm. People here understand me, but one really annoying thing is that they tend to look at my lips rather than my eyes when talking. If it's at all possible to teach an old dog new tricks, I should probably look for an elocution teacher and work on that accent,
* This is not the "Castellano" most Latin Americans speak, it's got all the lisping, uses vosotros, sois, and so on.