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After much experimentation, I suggest the new patch.
Benchmark results (time of replacing 1 of n character (ch1 to ch2) in 100000-
char string).
Py3.2 Py3.3 patch n ch1 ch2 fill
231 (-13%) 3025 (-93%) 200 1 'a' 'b' 'c'
626 (-18%) 2035 (-75%) 511 2 'a' 'b' 'c'
444 (-26%) 957 (-66%) 327 5 'a' 'b' 'c'
349 (-30%) 530 (-54%) 243 10 'a' 'b' 'c'
306 (-40%) 300 (-38%) 185 20 'a' 'b' 'c'
280 (-54%) 169 (-23%) 130 50 'a' 'b' 'c'
273 (-62%) 123 (-15%) 105 100 'a' 'b' 'c'
265 (-70%) 82 (-4%) 79 1000 'a' 'b' 'c'
230 (+4%) 3012 (-92%) 239 1 '\u010a' '\u010b' '\u010c'
624 (-17%) 1907 (-73%) 518 2 '\u010a' '\u010b' '\u010c'
442 (-16%) 962 (-62%) 370 5 '\u010a' '\u010b' '\u010c'
347 (-5%) 566 (-42%) 330 10 '\u010a' '\u010b' '\u010c'
305 (-10%) 357 (-23%) 275 20 '\u010a' '\u010b' '\u010c'
285 (-26%) 241 (-12%) 212 50 '\u010a' '\u010b' '\u010c'
280 (-33%) 190 (-2%) 187 100 '\u010a' '\u010b' '\u010c'
263 (-41%) 170 (-8%) 156 1000 '\u010a' '\u010b' '\u010c'
3355 (-85%) 3309 (-85%) 498 1 '\U0001000a' '\U0001000b' '\U0001000c'
2290 (-65%) 2267 (-65%) 800 2 '\U0001000a' '\U0001000b' '\U0001000c'
1598 (-62%) 1279 (-52%) 612 5 '\U0001000a' '\U0001000b' '\U0001000c'
1313 (-60%) 950 (-45%) 519 10 '\U0001000a' '\U0001000b' '\U0001000c'
1195 (-61%) 824 (-44%) 464 20 '\U0001000a' '\U0001000b' '\U0001000c'
1055 (-59%) 640 (-32%) 434 50 '\U0001000a' '\U0001000b' '\U0001000c'
982 (-55%) 549 (-20%) 439 100 '\U0001000a' '\U0001000b' '\U0001000c'
941 (-56%) 473 (-12%) 417 1000 '\U0001000a' '\U0001000b' '\U0001000c'
On other platforms other numbers are possible. Especially I'm interested in 
the results on Windows and on 64-bit. For the test I used the script 
replacebench2.py, and compared the results with the help of script 
https://bitbucket.org/storchaka/cpython-stuff/raw/default/bench/bench-diff.py 
.