I cant tell you about old aircraft, but I can tell you about operating in China. The metric altitudes translate into feet, and most airlines give pilots a conversion table. eg FL295, 315, 335, 354, 374 etc for 9000, 9600, 10200, 10800, 11400 metres. The only problem arises when cleared onto a level to intercept final app, where its not on your table and you need some kind of display to cross-check - but you only get this in smaller airports. Normally you intercept at 600m (2000ft) or 900m (3000ft).
That said, even on modern aircraft the indicated metric altitude may not read desired altitude eg (8990m not 9000m). Ie its a bastardisation, and reflects the fact that Chinese built aircraft still fly on metric altimeters whereas the rest dont.
One note of caution, China is going RVSM on November 22nd, and the altitudes are all X,100ft. The AIP says everyone shall set the altitude on their feet scale to prevent TCAS warnings.
One last point - the Russian conversion tables are different to the Chinese ones. Go figure.