I'm not a flooring expert, and I'm just replying mainly to bump your answer up to see if anyone else will chime in. I think usually the answer would be to modify your door transition strips to fit the new floor. You'll need appropriate barriers to your subfloor before laying the engineered planks, but we'd need to know more about your subfloor before advising on that.

Thanks! The subfloor under the existing solid hardwood is plywood (upper level). i am more worried about the gaps between flooring and door jambs.

Is there a reason why you are going with the more expensive 'rip-out and replace' option rather than the financially friendly sand/refinish?

Water based polyurethanes now offer LOW VOC (or super-low VOC) content. They are clear and colourless (they do not turn orange) and the cure in a VERY short time (5-7 days=full cure).

I'm hoping that you get a response to your question because I have a similar dilemma...I want to install LVP in a room that for some odd reason the original owner made DIAGONALLY half carpet (on slab) and the other half ceramic tile. There is about a half-inch difference between tile and slab so I was wondering if 1/2" cork underlayment would be okay to keep flooring level. Alternatively, installing cheap tile over the slab section. f*Tearing out the ceramic isn't an option because it is throughout the kitchen, bath, hall and dining room. Sigh.

You cannot use underpad underneath vinyl. You CAN use 1/2" cork under LAMINATE. See the difference? Vinyl is NOT allowed cork...laminate it is allowed.

My suggestion: sand/smooth/prime/fill the tile (yah...you have to do all of that) and then use a 12mm LAMINATE (no underpad attached) and 1/4" cork (total thickness = 3/4") to match the height of the tile. Remember to find a good t-mold that matches the LAMINATE.

It doesn't sound like you need the expensive 'water proof' property of vinyl so why limit yourself to the side-effects of vinyl?

A friend of mine had the same issue and her painter actually caulked underneath the door and you'd never know there was a gap once he painted over top

Plywood would be the best option the way I see it, or use a 3/4" thick engineered. Same thickness as the old. No messin' with door casings and the like.

3/4" engineered hardwood

The answer to you question is: you can‘t use engineered flooring. The height difference means you must go with real hardwood and tile the concrete area. Think about wanting to raise your ceilings to install taller crown height that you love. You just can’t.

We looked at a $2M house that offered engineered flooring over the expected site-finished hardwood and for me, it immediately made the house seem cheap. I don‘t care for the faux finish on engineered wood or the sound made when heels walk on it. I also don’t see where repairs are easily made. Engineered makes for a faster install, no fumes, no cure times, sustainability, and a large variety of widths. All of that works in a new construction environment. But keep in mind, there too, you cannot revert back to real hardwood. What is a homeowner to do?…raise all the cabinets and doors? Obviously, no.

I definitely would not chaulk the gaps. That is a maintenance nightmare for routine cleaning and changes in humidity and settlement. Choose real hardwood and tile the concrete area. You will be happy you did and you will love it forever.