Replace and Change Text

Change case/text with tr
~	swap lowercase $\Leftrightarrow$ uppercase at cursor pos
:.!tr '[a-z]' '[A-Z]'	swap current line to uppercase (mandatory spaces)
w~	change case to next word (capitalize)
b~	change case to previous or current word (capitalize)
:.!tr '{}' '()'	change curly brackets to parenthesis
Overstrike
r	replace one char at cursor pos
nr	repeat n times the char to be replaced at cursor pos
R	replace text until <esc> at cursor pos
nR	insert n times text replaced until <esc> at cursor pos
Insert
s ns	replace one or n chars with text until <esc> at cursor pos
S	delete current line and replace text until <esc>
Change Lines
cc ncc	change current or n lines beginning at current line
cG c1G	change all lines starting at current line to EOF or BOF
cnG	change all lines starting at current line to line n
	(forward or backward depending on pos of line n relative
	to current line)
c- nc-	change current and preceding or n lines
c+ nc+	change current and following or n lines
C c$	change all chars from current cursor pos to EOL
c0 c|	change all chars from column 1 to char preceding cursor pos
Change Words
cw ncw	change from cursor pos to end of current or n words
cb ncb	change from nearest or n preceding BOW to char before
	current cursor pos
Change Text Sections
c)	change from cursor pos to next EOSentence
c(	change from preceding SOSentence to char before cursor
c}	change from cursor pos to next EOParagraph
c{	change from preceding SOParagrapf to char before cursor
c]]	change from cursor pos to next EOSentence
c[[	change from preceding SOSection to char before cursor

Changing text such as words, sentences or paragraphs is not restricted to the current line. If the number of specified objects exceeds current line contents, the change operation is extended until the text specification is completely satisfied.

Case is changed by the vi built-in command ~ that swaps case. The tolower/toupper functions are achieved by the tr command in ex mode.
N.B. tr applies to the current line or to any specified line range, like in the following ex example:
:24,25!tr '[a-z]' '[A-Z]'
therefore to change the case of single words the ~ command must be used with an appropriate repeat count.

No vi or tr function is used to capitalize words, thus the vi combined commands w~ or b~ must be used to change case of word initials in forward or backward direction.

Overstrike operation is achieved with n r that overwrites the current character(s) with the charcater typed after r. More chracters are overwritten up to <esc> by R. n R oversrtikes n characters and appends n times the typed characters.
While R overstikes the following characters, s overstrikes (substitues) only the cursor charatcer and inserts the following one up to <esc>.
Finally S substitutes the current line in the same way as cc.

Examples of r, R, s, S
brrend a quite full line	current line of text
0rA	change first char to A
2rp	change next two chars to rr
Append a quite full line	current line after changes
02w2Rvery	change quite to very very
Append a very very full line	current line after changes
02sPre<esc>	change Append to Prepend
Prepend a very very full line	current line after changes
SThe end<esc>	change whole line with S, same as cc
The end	current line after line subsitution
n.b. - In the above examples $\sqcup$ marks mandatory spaces.

Change Text Sub-sections
cfc cnfc	change from cursor pos to first or nth occurrence of c on
	current line forward to EOL until <esc>
cFc cnFc	change from cursor pos to first or nth occurrence of c on
	current line backward to BOL until <esc>
ctc cntc	change from before cursor pos to first or nth occurrence of
	c on current line forward to EOL until <esc>
cTc cnTc	change from before cursor pos to first or nth occurrence of
	c on current line backward to BOL until <esc>
Change Pattern Matching Text Sub-sections
c/pattern<ret>	change from cursor pos to first occurrence of pattern
	forward to EOF (not including pattern ).
	If search wraps to BOF before pattern is matched, change
	begins with pattern and all text is removed up to, but not
	including, current cursor pos.
c?pattern<ret>	change from cursor pos to first occurrence of pattern backward
	to BOF. If pattern is matched before BOF is reached, change
	starts with pattern up to, but not including, current cursor
	pos. If search wraps to EOF before pattern is matched, change
	begins with cursor pos up to, but not including, pattern .