Start with a font you may convert
Keep the original font and its supplied terms. Choose the target application and format before changing the resource. The fact that an editor can open a file does not itself establish permission to modify or redistribute it.
Use the documented generation workflow
FontForge’s documentation describes opening an existing font and generating the desired output format. Its current documentation identifies File > Generate Fonts as the output command. Keep the editable source separately from the generated resource so one export does not replace the only working copy.
Account for outline and feature differences
FontForge’s FAQ distinguishes quadratic TrueType outlines from cubic PostScript outlines and describes additional conversion work between them. It also notes that Apple Advanced Typography and OpenType do not have completely overlapping capabilities. A new extension does not guarantee an identical feature set.
Specify what the output must preserve
Record the characters, styles and behaviors needed by the destination. A decorative heading and a multilingual text system place different demands on the resource. Use the font-file guide for format background and the font-editor guide for the broader authoring workflow.
Your questions, answered.
Is renaming .ttf to .otf a proper conversion?
No. A format conversion generates the required resource structure; changing a filename does not perform that operation.
Can every font feature survive every conversion?
No. FontForge documents differences between outline and layout systems and notes that some conversions lose information.
Sources & background
Sources supporting this page’s explanations. Practical examples are our editorial suggestions.