*Convert to up stair*
That means they can get up, but not back down.
Are you certain because if i interpret the last two responses to my thread that is not correct.
Also according to this pictogram
http://dwarffortresswiki.org/index.php/File:DF_Terraform.PNG on the wiki every level consists of a block adn then a tile entity on top of that block.
The down stairs compoment of the up down staircase(not sure if it's the same for the down stairs) will occupy the tile while the up component of the up down stairs will occupy the block or main component.
What i did was remove the tile entity or the down component of the up/down stairs, replacing or converting to and up stair.
Also it says on the wiki "These have nothing to do with the direction of travel, but only where they connect to and from. (To understand how stairs work and don't work, first make sure you understand how z-layers work.) " your response suggests that some polarity exist with respect to how stairs work with respect to entity movement which is very confusing and under my interpretation of up down stairs wiki seems to be in conflict with this statement.
Anyway as of now the updown is gone and an up is in it's place. One Y or Z layer below( i tend to think as Y as altitude or depth) there is an up/down but i figure i sealed it off.
I cannot seem to check with "k" if a floor tile exists so i must assume by converting updown to up only the floor tile has been sealed.