preventive timing chain replacement
Most members wait for a warning sign (rattle) before changing the cassettes because of the expense ($1500 to $2500 for a shop). Unfortunately, sometimes a cassette fails with no warning while driving down the highway and pistons and valves collide. When that happens it may be cheaper to replace the engine with a salvage engine rather then repair it. However, even the 2002 and later models (with the timing chain improvements) that has more than 200K miles is a candidate for cassette failure.
If the engine is being removed for any reason (low compression, bearing failure, etc.) and you plan to repair that and keep the vehicle for several more years I suggest replacing the cassettes while the engine is out - especially if you do your own work.
If the engine has no known major problems and no timing chain rattle it may not be cost effective to replace the cassettes as preventive maintenance.