I just did mine I got for free last week. Yep dirty. The change over would not budge due to all the animal hair and human, could not get it out without total teardown.
Word of warning if you do it, watch for the two screws that hold the microswitch in. There is one screw on the small cover and its not problem and safe to remove.
But once exposed there are two more and the top screw holds the other end of the foot pedal spring behind the microswitch plastic casing. Once that screw is off the spring will release (as will the microswitch when its cover is removed) and possibly the small plastic piece that was held on with the top screw that the spring attaches to in behind, on the backside. Reassembling- Putting the foot pedal spring back on, remove it from the pedal and put it on behind the microswitch first.
Other than that it was not too hard to do. Tho last two I did were DC25's and I went in expecting similar and they are quite different.
In the DC18's favor the wiring plastic loomy thing (sorry for not id'ing with right terminology) does not need to be removed to get the motor out. Bonus. The loom is under the motor behind the plastic casing and all one needs to do is disconnect the two leads into the motor.
I blow all motors and filters out with air compressors. Figure that far into it, might as well go lil bit extra.
oh..As to that clear plastic foot plate, try to be gentle. I think I broke mine with all the maneuvering with the other two pedals, tho perhaps it was ready to go anyways (I like to tell myself ..nah...wasnt me who broke it
There is a partial teardown on site by markcorrado1 https://manchestervacs.co.uk/DysonForum/index.php/topic,408.msg4636/highlight,dc18.html#msg4636
and also here Helpful Colin https://helpfulcolin.com/?s=dc18
Other than that I could find no other info on line when I was working on mine and ran into the spring attach issue behind the microswitch - that darn lil piece of plastic and where the heck that piece was supposed to go back into was a pain.
It's worth the fix. I love this DC18 and prefer it over the DC25 even tho it has less air watt, it is impressive and waaaa..aay more agile and easy to use.
addendum, you could have a look at Beko1997 teardown on the DC25, He replied to you above. If I remember correctly he has photos here on site and utube vid also. The models are not exactly the same, but if you haven't been inside a Dyson before, there is lots that is similar and will give you an idea about the foot pedal spring and the change over valve. General idea about it all and then figure it out from there. The DC18 is easier as you will not need to muck with putting wires back into the motor loomy thing, so don't let that deter you when watching Becko's teardown.
Beko maybe you need to teardown that DC18 you have & post a lovely photo montage and a vid like your awesome DC25 resources