Why don't you tell us about your mistake?  You will get better advise that way and we all make them.  On my first boat (CH17t), I glued the side panels together with the bow pointing up and the stern pointing down.  I doubt that you messed up worse than that!

Without knowing what you are trying to do, I would say that generally, bulkhead placement is not at all critical as long as it is cut so that it does not deform the hull.  Some designs don't specify bulhead location and some boats are built either without BH or with (non-structural) foam bulkheads to save weight.  What is important is that you keep the seat in the same position as specified.  If you try to move the cockpit and seat aft that much, you will certainly alter how the boat handles.

The kayak sanded smooth, ready to be fiberglassed. Nervous and excited.  I was getting all the neccessary items to begin the milestone, ie: plastic covers, brushes, paper towels and the like, including the warming lamp.  Well I placed the lamp (the shop kind that has a metal shade to direct the heat better), well I placed it ontop of the deck as a table while I get more supplies.  I left the room to go get more suppllies.  Around the corner I plugged in the extention cord for the light.  Well I didn't realize the other end was connected to the heat lamp.  As I came back in the room  I begin to smell burning.  I quickly noticed my fault and removed the lamp.  Yep it almost cought on fire!  Ambers were lit in the wood. It left a chard, cracked spot of burnt, smoking, circle about 3.5 inches wide.  I had to throw water on it or it would have actually catch on fire.  I think I cried, not sure.  lol.  So how do I fix this?  I didn't want to cut out the circle and rebuild that area because I don't have the matching wood for that area.  CLC sent me my wood with almost every strip of different shades.   No way to match it.  So I thought make a day hatch in that area "not planned until now"  I'll buy an after market day hatch.  A 4 inch is too small and a 6inch is fine but the outer lip reaches just over 8.  It happens that the aft bulkhead sitts just about an 1inch forward within the border of the burnt spot.  To continue I think I should move the bulkhead aft in order to clear the area where the day hatch would be plced and allow it to be in the same compartment as the cockpit.  So do you all see how embarrassing this is? After 60+hours to build the strips and fair them out, sand them smooth, ready to FB and I sabotage my work.  Very sad.

Oh bummer.  Sounds like Mr Murphy wants to make sure that you have a custom kayak! 

No doubt that your day hatch idea will work fine and will be functional.  The only negative is that with the bulkhead moved aft, you will have a bit more water in the cockpit if you tip over.  Not really a big deal but it may take a bit longer to pump out. 

If it were me, I would go with the cosmetic approach.  CLC has a number of onlays that are big enough to cover the area.  I am actually using the "Built By" onlay to cover a very ugly staple mark on the aft deck of my current build.  Another choice would be to go with a piece of fabric like many put on the CLC Kaholo SUP.  If you go with the fabric idea, get two pieces and test one for color steadfastness with epoxy before glassing onto the deck.  I might suggest that you use a patch from your local fire department to honor the day that you saved your kayak from fire!  (I mean no disrespect here, just trying to interject some humor!)

Here is a patch that I put on the bow of my wife's Ch16LT (her childhood nickname was Tweety).  I am the only one who knows what is underneath.

I have to admit.  I needed that.  Very funny and completely apporpriate.  Also a good idea.  But I think I will stick with the day hatch.  I actually think the 4 inch will do.  That does bring up anyone's opinion....Should the day hatch enter into the cockpit or aft of the bulkhead?  If I have the day hatch placed beyond the bulkhead that would then make me bring the aft bulkhead foward an inch instead of abaft 4".  So again if I place it back more then that would position the day hatch to open into the cockpit.  If I move the bulkhead foward then the day hatch would open into the aft compartment.  What do you prefer?