I would like to know if it is possible to complete mechanical engineering as an undergraduate and then pursue aeronautical engineering as a masters degree?
Does mechanical engineering have a better scope than aeronautical engineering in the future?
What are the possible jobs that a mechanical wengineering can take up?
What is the salary range for a mechanical engineer?

Absolutely - mechanical engineering can provide an excellent base for pursuing a graduate degree in Aeronautical Engineering. I would recommend that in your junior and senior years you take electives in aeronautical engineering specialties, as well as fluid mechanics and structures - this will help you see whether aeronautical engineering is truly what you want to do. As for whether an undergraduate degree in ME is "better" - well, that depends on you. As an ME you have the option of deciding as a junior or senior to concentrate on areas such as mechanics, fluids, heat transfer, bio-mechanics, aeronautics, etc, so you don't get locked in until you have a better idea of precisely what you want to specialize in.

Typical ME jobs include professions such as machine design, heat transfer analysis, structures, aeronautics, automobile engineering, tribology (i.e. wear and erosion), etc. I have both a bachelors and masters degree in ME and have spent my career in designing electrical and fiber optic connectors and components for telecommunications.

Starting salaries depend greatly on your particular specialty and the area of the country. Depending where you live typical MEs with BS degrees start at around $50K, and around $70K for masters. Experienced engineers are around $90K - $110K, and engineering managers are in the $100K - $130K range.