Extend and trim 3D polyline although keep z value constant

I figure the above is possible although I'm not sure ho to do it.

Say I have a 3D line (actually I have many, but lets keep it simple). X1, Y1, Z1 = 0, 0, 0; X2, Y2, Z2 = 10, 10, 10

I am in plan view looking down at a 2D representation of the 3D line. I decide I want to trim (or extend) the polyline. I draw a crossing line and trim the line, so that X1, Y1, Z1 = 0, 0, 0, and X2, Y2, Z2 now = 5, 5, 5

How can I maintain the original z value at the extended or trimmed location? E.g. I want my trimmed line to have X2, Y2, Z2 values of 5, 5, 10?

Let me know if I have explained it so badly that no one understands

For your simple example you could, grip edit the end of the 3Dpoly using .Z point filter and Apparent Intersection OSnap.

If it's something you do regularly, you could automate the procedure with a macro/lisp

Also, you can grip edit the endpoint to the Apparent Intersection while OSNAPZ=1 and the grip's Z will be maintained.

A suggestion for a lisp when you pick the line you do so near to one end this then imply's which Z is to be maintained you can do a quick check for end picked trim line then update the Z.