The story of my case was that I had what was called a "superficial" leg vein clot and was sent home from the lab and ER, only to have a PE less than two months later. After the PE they found that the clot had spread through the popiteal to the superficial femoral before leapfrogging past the deep vein and vena cava to the lungs.
11 months out, I learned I had been carrying Factor V Leiden Heterozygous, and understanding how FVL worked in turn explained how the clot grew, spread, and just did not get arrested without anticoagulation.
Apparently a clot can grow backwards due to slowed circulation, or can grow its way downstream toward the heart. Everyday reasoning probably directs thinking of a clot as being some kind of lump (which it often is for starts) instead of being some kind of long streak, but the latter of those two forms is often what happens, especially over time.
I am now over a year out from having that long streak (in addition to the PE) identified and I am obviously still alive. A follow on scan unfortunately did not show the situation to be much improved. However, there was an earlier poster who had similar damage and in the course of his follow on scans, some incremental improvement was seen between the 1 1/2 and 2 1/2 year marks.
I suppose it matters where the clot is; everything matters. What matters more is the chance the clot will break off and lodge somewhere. Older clots are less likely to break off. They may become chronic clots, which are somewhat calcified and stable, and ingrained into the vessel where they do less harm. Such chronic clots may be fertile ground to grow new clots if you leave anticoagulation. Some, if not most, of the damage is irreversible. The body does not regenerate damaged valves in the leg veins. All we can do is stay mobile, stay anticoagulated as needed, and wear compression socks.