If they've got paper and ink they can create whatever they wish.
for the euro to take hold, there had to be a period of overlap cash in where old currency could be turned in. Stores would not accept dracma and only banks would convert.
To reverse it, you would have a massive black market in dollars and some euros. You can’t shift to a new currency with an old currency (euro) in circulation.
Hence my use of “functionally”. Sure they can print drachmas, but it will not function as currency. There isn’t even a pretense of value behind it. The dollar once was gold/silver-backed and weaned away from that when other value was infused. The euro was based on a wholesale agreed-on currency swap, eliminating certain currencies and replaced by fixed ratios with something new and otherwise valueless. The drachma, however, will have no basis in any value real or implied - save only perhaps by demanding tax payments in drachmas, thus persuading people to accept government checks issued in that otherwise valueless currency.
How many slices of bread can you buy for a wheelbarrow of cash in Zimbabwe?