The Chief of the Defence Staff has told Forces News the size of the military "might" still shrink through natural turnover as career structures "evolve".
General Sir Nick Carter said as the Armed Forces transition from an "industrial age military" to "an information age military" there will "clearly" be changes for personnel.
His comments come after the Prime Minister announced an extra £16.5bn will be given to the Ministry of Defence over the next four years.
Speaking to the House of Commons on Thursday via video link from Downing Street, Boris Johnson promised there will be "no redundancies" in the Armed Forces and vowed to strengthen the UK’s cyber capabilities, developing artificial intelligence (AI) technology and continuing with its space programme.
Gen Carter said: "Over the next 10 years, as we evolve from being an industrial age military that is dependent upon platforms to an information age military that is more about systems, clearly that will have an impact on the sorts of people we need to employ, the way we train and educate them and the sort of makeup of those people."
He added "choices have yet to be made" over the potential downsizing of personnel numbers but also said: "At the heart of everything that we will do hereafter are our people because ultimately, they represent our adaptive edge."
When asked if the funding signals an abandonment of traditional military hard power, Gen Carter told Forces News: "We’re going to invest in modern hard power.
"Whether it's space or cyber, these are the modern capabilities that are necessary, when integrated with other domains like maritime, air and land that give you the total effect which is modern hard power.
Watch: Prime Minister Boris Johnson announced the additional defence spending to the House of Commons via video link on Thursday.
"It certainly does not imply we are disinvesting from hard power."
He added: "If you are going to modernise… some capabilities will have to meet their 'sunset', while others emerge for the 'sunrise'.
"The trick is to make sure the distance, 'the night', if you like, between sunset and sunrise is not too long."
Gen Carter said "choices will have to be made", adding: "This settlement gives us the opportunity to think very radically I would suggest, but thoughtfully about how we do this modernisation, how we limit 'the night' and how we modernise our Armed Forces to be appropriately organised and equipped for warfare in the 2030s."
Gen Carter added service personnel can expect to hear more about future plans "during the course of the next few months".
The funding boost is on top of the Government's manifesto commitment to 0.5% annual increase in real terms.
Mr Johnson said the investment will result in a "renaissance of British shipbuilding" across the country, vowing to "restore Britain’s position as the foremost naval power in Europe".
In regards to the Army, he added: "We shall reshape our Army for the age of networked warfare, allowing better-equipped soldiers to deploy more quickly."