Will the new OpenType font be compatible to my old Type 1 or TrueType font?
No, although the design of the OT font may the same as the older Type 1 or TrueType there are many subtle changes to kerning, line spacing and naming of the OT font that dont allow compatibility between the old and new fonts. The extension of the glyph set and much broader support for languages prohibit any type of backward compatibility between the old and new fonts.