If we take “old allies” not so much as old friends but more as “previously allied with:” Japan was on the side of the allies in WW1 but was axis in round 2. Nazi-Germany invaded the USSR in spite of their peace pact. Napoleon and tsar Alexander of Russia were on somewhat friendly terms before Alex clandestinely rejoined the coalition against the French.
If we take “old allies” not so much as old friends but more as “previously allied with:” Japan was on the side of the allies in WW1 but was axis in round 2. Nazi-Germany invaded the USSR in spite of their peace pact. Napoleon and tsar Alexander of Russia were on somewhat friendly terms before Alex clandestinely rejoined the coalition against the French.