Why there is a line
Once most of a pallet is new wood, the original treatment mark says little about the wood actually in it. ISPM 15 therefore draws the line at about one third of components: below it the pallet is repaired, above it remanufactured.
What has to happen
Previous marks must be permanently obliterated, for example by covering with paint or grinding. The unit is then treated again and marked anew. Remanufactured wood packaging can combine reworked components from several old units.
For euro pallets
A heavily rebuilt pallet in the 1200 × 800 footprint is not an EPAL pallet unless an EPAL licensee rebuilt it under licence, and in practice it should not carry EPAL markings at all. Many recyclers break such pallets down and sell the timber instead.
Practical test
Count replaced parts. On an EPAL 1 with 11 boards and 9 blocks, replacing more than six or seven of those 20 components puts it past the threshold.