While I agree that if you're going to transplant that tree, it'd be
faster, easier and cheaper to transplant it while it's young and small
than when it's fully grown.
But, I disagree that it's likely to be a problem.
You see, regardless of how tall a tree is, and how large it's crown,
99.9% of it's root network will be in the top 3 feet of soil around your
house. This is because a root's job is to anchor the tree, collect
water from the ground and absorb nutrients the tree needs to grow. And,
those nutrients come from AEROBIC decay of plant material, which is what
you get when you compost leaves or grass or other organic biomass.
Generally, anything deeper than 3 feet in the ground won't have enough
oxygen to produce aerobic decay, and all you get is anaerobic decay that
produces methane gas, and the tree is not interested in that stuff at
all. It wants the good stuff that rots near the surface.
It's the fact that a tree's roots are only interested in the aerobic
decayed biomass that's the reason why you can have a hundred thousand
fully grown elm trees growing on the lawns and boulevards of a city like
Winnipeg, and yet tree roots growing into sewer lines is comparatively
rare. That's because the sewer lines are below the frost line depth of
about 6 feet, and the roots of the trees won't normally grow to that
Where you have a cracked sewer pipe, the roots will sense the
aerobically decayed biomass getting into the soil and grow toward it.
In that case, you will have roots growing down 6 or 7 feet deep to get
at that aerobically decayed stuff. But, it's been my experience that a
plumber simply cutting those roots with a snake will solve the problem.
The root end dies and you don't get the tree trying to grow more roots
into that sewer pipe.
Also, you should be aware that the root network of a tree will typically
be about 10 times the diameter of the crown. So, if a tree's branches
extend out 20 feet from it's trunk, the root network will extend out 200
feet from the tree. It's that huge amount of soil the tree grabs onto
that allows it to withstand hurricane force winds.
Maybe Google "Tree Roots" and learn all about them. Most people imagine
that the root network of a tree is a mirror image of the tree's branch
network (aka: the "crown") and nothing could be further from the truth.
It's knowing that difference that will help you make a decision on what
to do with that tree and when to do it.