Its normal (in the sense that dried up paste will stick to the cpu) i did this alot back in the day and just plug it back in as long as no bent pins, Next time twist the heatsink back and forth before you remove so it dosent happen again. Because you had...
Finally, in order to avoid the reapplication of thermal paste, can one replace the fan only while the heatsink is still screwed to the main board by sliding the fan sideways, or are there obstructing parts which make it necessary to lift the whole fan heatsink assembly off of the mother ...