AFAICT, re-enabling these old OP-codes would require a hardfork.

If we had SegWit enabled, we could via a soft fork allocate new OP-codes
for the same functionality (by introducing a new version of Script).
I believe the Elements alpha project has been experimenting with
re-enabling old OP-codes: https://elementsproject.org/elements/opcodes/

2017-05-19 8:07 GMT+02:00 Mark Boldyrev via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org>:

> Back in 2010, there was a bug found in Core which allowed
> denial-of-service attacks due to the software crashing on some machines
> while executing a script - see CVE-2010-537.
> I believe the removed ("disabled") opcodes should be re-introduced along
> with a standardized behavior definition.
> For example, when execution of an opcode results in an arithmetic error,
> such as OP_DIV with a zero divisor, the script should exit and fail.
> The string splice opcodes should also check their arguments for
> correctness, etc.
>
> These opcodes would enhance the flexibility of scripts and allow
> sophisticated native smart contracts to be created.
>
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