It should not break your existing tables.
It just adds colspan support to TABLE BB code.
You can use it like that:
Code:
[TABLE]
[TR]
[TH]One[/TH]
[TH]Two[/TH]
[TH]Three[/TH]
[/TR]
[TR]
[TD colspan='2'][CENTER]Un[/CENTER][/TD]
[TD]Deux[/TD]
[/TR]
[TR]
[TD]1[/TD][TD]2[/TD][TD]3[/TD]
[/TR]
[TR]
[TD]4[/TD][TD]5[/TD][TD]6[/TD]
[/TR]
[/TABLE]
You can't merge table cells directly from the HTML/WYSIWYG editor. You need to switch to the BB editor to adjust your cells.