I just had this problem myself. When the FBK was imported, all of the coordinates were unknowingly scaled down to reflect the unit conversion. The first thing you should do is check how the coordinates in your drawing compare to the coordinates in your FBK. If your FBK is just a series of angles and distances, then you might want to re-run it through a dummy drawing that has both units set correctly. If your coordinates are bad, don't panic, because it may be a simple fix. Here's what I had to do:
Export your current "Allpoints" point group as a point file. Export your surface and all other entities (alignments, etc.) as XML files. You could export the whole drawing as one file, but I found it to be more useful to export each one separately. Turn on/thaw/unlock everything. Also, if you have any styles that are set where everything in the display is off, edit the style so something is on. Scale your entire drawing using 0,0 as the base point by a factor of 1.000002. It's important to make sure everything is displayed because you can only scale what is visible. This should adjust the coordinates to the right units. This factor is the International Feet to US Survey Feet conversion. Before you scale your drawing, check a few points first by multiplying the northing and easting coordinate in the drawing by this factor and seeing if it gives you your correct coordinate. If not, you may need to just calculate a new factor.
When I scaled my drawing, everything was adjusted except for Survey Figures. This caused a problem in the surface because some Figures are treated as breaklines, so I had breaklines that did not scale, and the contours showed it. To fix this, I deleted the surface, imported the surface XML, and then scaled just the imported surface. This worked because the XML doesn't recognize breaklines, just the TIN coordinates. Note that things are scaled vertically as well as horizontally. Unless your elevations are really high, this shouldn't matter. Mine were in the 700' range, which meant a difference of only about 0.0014'. If you want, you can shift your surface vertically. My northings and eastings were around 614000 and 1470000, causing about a 4' shift, which was bad.
To fix the survey figures, I imported them into a dummy drawing, exploded them into 3D polylines, exploded them again into 2D polylines, set their elevations all to 0, scaled them, then copied them into my real drawing. Be sure you go through and delete your old figures after the new polylines are in. This isn't the greatest, because their elevations have all been removed, but they graphically appear as they should. And the only lines where elevations matter are the breaklines, which have already been taken care of.
Hopefully that should be the end of it. I had another problem in my project because I imported just a few points as a point file as opposed to an FBK. These points were already correct because they didn't go through the survey database, so when I scaled them, the coordinates were bad. I just re-imported them and overwrote their scaled location, or you could just select those points and scale by 0.999998 to get them back to their original location.
The best advice I can give you is to check multiple things often throughout the process, and keep a backup of everything before you modify it. The best and last check to perform is to export your Allpoints point group after everything is done and compare it to the one you exported originally before any adjustments were made. Set up a spreadsheet to calculate the difference in northings, eastings, and elevations. Every difference should be pretty consistent throughout.
Good luck! Hopefully your problem is a little easier than mine was.
One other thing I just remembered: Make sure your XML files are bring imported/exported as US Survey Feet. To check, open the XML in a text pad/note pad program, and look at the top where it will either say "linearUnit=foot" (international) or "linearUnit=USSurveyFoot". This is also where you can change the text to fix it if it's wrong.