How to change the delay variable Td for a tline component in LTspice?
tline and ltline models have fixed delay, i.e. they cannot be a function of time. If you need such a delay, you can only use a behavioural source with the builtin function V=delay(V(in), f(time)) (for BV source, for ex.), where f(time) is a function defined previously with a .func statement, or made up ad hoc (e.g. V=delay(V(in), V(control))).

You should know that, while you can use that, in both .AC and .TRAN analyses, in the latter it may suffer from minor (seemingly erratical) drifts, depending on the configuration of your schematic. One cause may be the numerical accuracy (dynamic range) which, unfortunately, affects the behavioural sources, but I can't point out exactly what/who/where, I'm afraid that's up to you to find out.

Alternatively, you can build your own LC delay line built with the behavioural inductors (Flux=f(x)) and capacitors (Q=f(x)), but I, personally, would recommend avoiding. Your choice.