Note that for simple *content* formatting, much of the same syntax still applies. //italics//, ''bold'', etc. are the same as before. Even the use of "|" to define tables is the same.
However, almost none of the plugins for TWClassic can be "converted" to work in TW5. This is because of fundamental changes in the underlying architecture of the TWCore code. In addition, there is no *automatic* way to translate the old TWClassic "macro" syntax into the new TW5 "widget" syntax, but the functionality of many of the more significant plugins is still available, just in a different form, which has often been directly incorporated into equivalent TWCore capabilities.
For example, the TWClassic "ForEachTiddler" plugin is now supported by the TWCore <$list>...</$list> widget. The <$list> widget uses a powerful new "filter" syntax to select which tiddlers to operate on then sets a variable, <<currentTiddler>> to the title of each selected tiddler. Within the "body" of the <$list>...</$list> widget, this variable can then be used to display or manipulate the contents of each matching tiddler.
Obviously, there is likely to be a considerable effort needed to re-implement old TWClassic functionality using new TW5 syntax.... but it is possible and, in my opinion, well worth the effort in most cases.
The best way to start the process is to tell us which TWClassic plugins you are using, and what essential functionality they provide, so that we can suggest modern TW5 alternatives and then offer some guidance and assistance in helping you learn how to re-write your old TWClassic content to work with TW5.
-e