Add Backwards Compatibility section to OP_CHECKTEMPLATEVERIFY BIP and change 'Implementations' header to 'Reference Implementation'

Author: JeremyRubin

bip-ctv.mediawiki

@@ -225,9 +225,9 @@ In order to facilitate using CHECKTEMPLATEVERIFY, the common case of a PayToBasi
 with no scriptSig data shall be made standard to permit relaying. Future template types may be
 standardized later as policy changes.
 
-==Implementations==
+==Reference Implementation==
 
-An example implementation and tests are available here:
+A reference implementation and tests are available here:
 https://github.com/JeremyRubin/bitcoin/tree/checktemplateverify.
 
 
@@ -519,6 +519,20 @@ Given the simplicity of this approach to implement and analyze, and the benefits
 applications, CHECKTEMPLATEVERIFY's template based approach is proposed in lieu of more complete
 covenants system.
 
+== Backwards Compatibility ==
+
+OP_CHECKTEMPLATEVERIFY replaces a OP_NOP4 with stricter verification semantics. Therefore, scripts
+which previously were valid will cease to be valid with this change. Stricter verification semantics
+for an OP_NOP are a soft fork, so existing software will be fully functional without upgrade except
+for mining and block validation. Similar soft forks for OP_CHECKSEQUENCEVERIFY and OP_CHECKLOCKTIMEVERIFY
+(see BIP-0065 and BIP-0112) have similarly changed OP_NOP semantics without introducing compatibility issues.
+
+Older wallet software will be able to accept spends from OP_CHECKTEMPLATEVERIFY outputs, but will
+require an upgrade in order to treat PayToBasicStandardTemplate chains with a confirmed ancestor as
+being "trusted" (i.e., eligible for spending before the transaction is confirmed).
+
+Backports of OP_CHECKTEMPLATEVERIFY can be trivially prepared (see the reference implementation)
+for older node versions that can be patched but not upgraded to a newer major release.
 
 
 == References ==