The orifice size is different on a 134a expansion valve and the power unit may be looking for a different head pressure. Short answer, yes, it'll work. Long answer, it's complicated. You may lose some capacity. If the R12 unit is still good, use it.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)