Whether the impact of phase two will lie at either of these extremes is not yet clear.
"Problems last time arose largely, I think, because reduced class contact time sucked up the supply pool," says Mr Dempster, "so there wasn't enough left to cover teacher absences. Heads and deputes had to take classes, which meant they had less time to manage their schools.
"We have concerns that the same thing might happen again, particularly if, as the HAS survey showed, local authorities have not fully funded the extension to reduced class contact time."
However, there are differences this time.
"The lack of supply cover has been addressed, to some extent, by the continuing increase in teacher numbers. So we are in a different position now from when the first phase was introduced," explains Mr Dempster.
"Reduced class contact time is a great thing for teachers and for education generally. So we are hopeful this phase might work out better than the last one."
A little flexibility from all professionals in Scottish education might be needed for the first month or so, says Mr Dempster.
"Teachers should not normally be expected to carry over non-contact time from one week to the next. But on rare occasions, if there is a big staff absence, that might be the best thing for a school, at least until we can all see clearly whether we do have to call for more resources."
The question of resources is one the teaching unions find difficult to disentangle, says Drew Morrice, of the Educational Institute of Scotland.
"On the one hand, you have the executive saying funding is in place for councils to meet the 2006 requirement. On the other, you have councils saying the money is not enough.
"We know councils are hard-pressed, but we would be concerned if pressures on them were allowed to affect their implementation of the national agreement."
Funding to local authorities over the next couple of years will be critical, says Mr Morrice.
"Like HAS, we were picking up evidence of contraction of budgets. There has clearly been some dialogue to resolve this, but not solely linked to delivery of non-contact time. My impression is it is connected with the commitment to move to 53,000 teachers.
"The executive can train the new teachers, but the authorities have to employ them. So if there's a bottleneck there, because of budget tightness, it's in everybody's interest to open that up."
Apart from funding, 2006 should not be as difficult as 2004, says Mr Morrice. "In fact, evaluation of what worked last time (such as visiting specialists) and what did not work (whole-school assemblies) should make it easier for us all this time around."