LODS should update ESI position and STODS the EDI. But they don't! (idon't know why) Sad so I did EDI update manually each loop.

004010A0  40 60 60 00 5B 7B 7B 00 5C 7C 7C 00 5D 7D 7D 00  @``.[{{.\||.]}}.
004010B0  5E 7E 7E 00 5F 7F 7F 00 C0 E0 E0 00 C1 E1 E1 00  ^~~._.Ààà.Ááá.
004010C0  C2 E2 E2 00 C3 E3 E3 00 C4 E4 E4 00 C5 E5 E5 00  Âââ.Ããã.Äää.Ååå.
004010D0  C6 E6 E6 00 C7 E7 E7 00 C8 E8 E8 00 C9 E9 E9 00  Æææ.Ççç.Èèè.Ééé.
004010E0  CA EA EA 00 CB EB EB 00 CC EC EC 00 CD ED ED 00  Êêê.Ëëë.Ììì.Ííí.
004010F0  CE EE EE 00 CF EF EF 00 D0 F0 F0 00 D1 F1 F1 00  Îîî.Ïïï.Ððð.Ñññ.
00401100  D2 F2 F2 00 D3 F3 F3 00 D4 F4 F4 00 D5 F5 F5 00  Òòò.Óóó.Ôôô.Õõõ.
00401110  D6 F6 F6 00 D7 F7 F7 00 D8 F8 F8 00 D9 F9 F9 00  Ööö.×÷÷.Øøø.Ùùù.
00401120  DA FA FA 00 DB FB FB 00 DC FC FC 00 DD FD FD 00  Úúú.Ûûû.Üüü.Ýýý.
00401130  DE FE FE 00 DF FF FF 00                          Þþþ.ßÿÿ.    

include 'win32ax.inc'


proc strToUpper str:dword

        mov   esi, [str]        ; define src
 .next: mov   edi, esi          ; define dest [and later, new dest]
        lodsb                   ; AX = a char
        cmp   al, 0             ; if end_of_string exit
        je   .exit
        mov   bl, al
        and   bl, 'a'
        and   bl, 'z'
        cmp   bl, 'a' - 1
        jnz  .next
        sub   al, 'a'-'A'       ; offset
        stosb                   ; loads AX to str
        jmp  .next
 .exit: ret
endp

proc strToUpper2 str:dword

        mov   esi, [str]        ; define src
 .next: mov   edi, esi          ; define dest [and later, new dest]
        lodsb                   ; AX = a char
        cmp   al, 0             ; if end_of_string exit
        je   .exit
        mov   bl, al
        sub   bl, 'a'
        cmp   bl, 'z' - 'a'
        ja    .next
        sub   al, 'a'-'A'       ; offset
        stosb                   ; loads AX to str
        jmp  .next
 .exit: ret
endp

start:
stdcall strToUpper, buff
stdcall strToUpper2, buff2

mov ecx, 256
mov esi, buff
mov edi, buff2
mov ebx, diffBuf

.loop:
test ecx, ecx
jz   .halt

repe cmpsb
je   .halt

lea  eax, [ecx-256]
not  eax

mov  dl, [buff+eax]
mov  [ebx], dl
mov  dl, [buff2+eax]
mov  [ebx+1], dl
inc  al
mov  [ebx+2], al
mov  byte [ebx+3], 0
add  ebx, 4

jmp .loop

.halt:int3 ; Call OllyDbg 

align 16
diffBuf rb 256*4

buff:
  repeat 256
    db % and 255
  end repeat
buff2:
  repeat 256
    db % and 255
  end repeat

.end start    

I think that way you proposed can't work in characters like Ç â é ã... and the AND way it works.

err.. I was testing and seems the only exception is characteres between #123 and #127. They change.

Coding it yourself is the best solution

...accept the challenge...

...building my own syntax highlight.

Is your boldness for sale ?

- You can find a good example of sy-hili in the FASMW source by Tomasz

Tips:
Imho One should begin from now:
- to think 64bit

...i will be very glad to adopt it in my biberkopf.

32bit will disappear fast?

No offence intended.

Is my monitor lying to me, or your selection's highlighting logic suffers from ubiquitous "off-by-one" mistake?

Why do you call that "outlining"? Maybe "collapsing" sequence of instructions to some descriptive comment?

Storage of arbitrarily long strings is performance vs. memory tradeoff. First decide how you will handle "undo" (you may store original file verbatim and dynamically apply consolidated changes, for example of memory-wise).