Just try! Use an alternative lowerrom with your CPC Plus. If you get the READY prompt you are done.
Plus cartridge is mapped at rom pages 128 and above, just mirrors the 128, 129 and 131 (not exactly sure) as lowerrom, basic and amsdos. if you have your own lower rom, basic and M4rom at the M4 rombanks and get the READY prompt just try to save the upper rom pages. I did long time ago a DSK for dumping in basic the lowerrom from system cart (To get spanish and french key layout). Can be modded to dump the rest of blocks... It's a modified version of ROM dumper example at Marco Vieth's CPCEMU 1.5 basic utils for real CPCs.
100 REM ROMGET (v2.0)
110 REM 3.9.1994
120 REM
130 MODE 1: CLEAR: DEFINT a-z
140 PRINT"ROMGET v2.0":PRINT"Run on a CPC to save the ROMs as files.": PRINT
150 PRINT"Insert disk with >= 50K free space": PRINT"and press a key ..."
160 CALL &BB18
170 adr=&A000: sh=adr+2: dh=adr+5: k=adr+15
180 h!=HIMEM: MEMORY &1FFF
190 FOR i=adr TO adr+&12: READ a$: POKE i,VAL("&"+a$): NEXT
200 POKE sh,0: POKE dh,&20: POKE k,&FC: CALL adr
210 POKE sh,&C0: POKE dh,&60: CALL adr
220 PRINT"Saving lower&upper-ROM (CPCXXXX.ROM) ..."
230 SAVE"!CPCXXXX.ROM",b,&2000,&8000
240 POKE k,7: CALL adr
250 IF PEEK(&6000)<>1 THEN PRINT"No AMSDOS-ROM found.": GOTO 280
260 PRINT"Saving AMSDOS-ROM (CPCADOS.ROM) ..."
270 SAVE"!CPCADOS.ROM",b,&6000,&4000
280 PRINT"Trying to find some other ROMs ..."
290 FOR i=1 TO 251:PRINT HEX$(i);
300 IF i=7 THEN 360
310 POKE k,i:CALL adr
320 IF PEEK(&6000)=&80 THEN 360
330 PRINT" ROM found -- ";
340 PRINT"Saving MYROM";HEX$(i,2);".ROM ..."
350 SAVE"!MYROM"+HEX$(i,2)+".ROM",b,&6000,&4000:PRINT
360 PRINT CHR$(13);:NEXT
370 PRINT:PRINT"bye."
380 MEMORY h!: END
390 DATA 21,00,C0,11,00,20,01,00,40,DF,0D,A0,C9,10,A0,00,ED,B0,C9