Looks good ,,now keep it empty for 8-10 weeks
Not sure i understand your questions about "making fish safe"
Whatever filters/sponges and/or rocks you have in the 20 will have to go in 40(you can even use a rock or 2 from your 28 gl, i keep an empty HOB
on my DT running for my QT,when i know i need to QT i put some floss in it for a few) .. .. You can either use the 20 gallons of water from the 20 (i would) or make all fresh..Are you going to do hypo??
Glad i could help..i went through this exact situation 6 years ago..i do remember the wait and staring at an empty DT,,but its so worth it..