It has been a while since I've heard this issue from a user. Back when ACA was the most popular conjoint method in the mid 1990s, people used to write us when they had committed this error (and that happened probably 3x per year). There is a way to recover and "mostly" set your data right, but there were some things that happened during the display of the survey to respondents that you cannot change.
Two main things happened that you cannot change:
1) When respondents saw the "Importance" question (assuming you used the default Importance question supplied by ACA), they would have seen the worst and best levels in switched order. Since you probably informed respondents to consider the difference between the extreme levels, this was likely not a problem.
2) The software uses the prior ranking and importance information to show pairs of product concepts that imply difficult tradeoffs (utility balance). When the level rank order was misinformed for an attribute, it's quite possible that respondents saw a few "dominated" concepts, where one product was clearly better than the other. This doesn't ruin the design or invalidate the results, it just makes the questionnaire design not quite as efficient as it could have been.
So, how to solve the problem? We have done it many times here for individuals but it is a bit tricky (we do this for a fee). But, if you have some good data processing skills and have access to the standalone ACA/HB system (not the one integrated within SSI Web), then you can do it yourself.
You need to look up the documentation (in the SSI Web manual) for the .ACD file. You need to export your .ACD file from SSI Web and modify that file (typically via a programming script) to switch around the way it was recorded that it displayed the Importance question (which level was best and which level was worst) ...plus the rank order of the levels within that attribute that are shown on the same line in that portion of the .ACD file.
Then, once you've corrected the .ACD file, you submit it for HB estimation using the standalone ACA/HB software system.
If you are already licensed to use the integrated HB estimation as part of your SSI Web ACA license, then you have automatic rights to use the standalone ACA/HB system that you may download from our website.