Well, i'm not too familier with Slackware (something I should look at soon), however, if your mount point for your cdrom is /mnt/cdrom , then then following commands should help:
QUOTE
cd /
umount /mnt/cdrom
...now your cdrom should be able to open, and note, your cdrom will not open for you if you still have it mounted. And, it will not unmount unless there is nothing using it which includes being in the /mnt/cdrom directory.
Replace the cd, and remount it using
QUOTE
mount /mnt/cdrom
If your mount points are different (for example, debian uses /cdrom rather then /mnt/cdrom) then just replace it above.