I think the change was made because many abbreviations were ambiguous.
In the US, AST is commonly used for Standard Alaska Time, 10h west of GMT.

See the entry in file NEWS of Release 2017a - 2017-02-28 00:05:36 -0800

  Changes to past and future time zone abbreviations

    Switch to numeric time zone abbreviations for South America, as
    part of the ongoing project of removing invented abbreviations.
    This avoids the need to invent an abbreviation for the new Chilean
    new zone.  Similarly, switch from invented to numeric time zone
    abbreviations for Afghanistan, American Samoa, the Azores,
    Bangladesh, Bhutan, the British Indian Ocean Territory, Brunei,
    Cape Verde, Chatham Is, Christmas I, Cocos (Keeling) Is, Cook Is,
    Dubai, East Timor, Eucla, Fiji, French Polynesia, Greenland,
    Indochina, Iran, Iraq, Kiribati, Lord Howe, Macquarie, Malaysia,
    the Maldives, Marshall Is, Mauritius, Micronesia, Mongolia,
    Myanmar, Nauru, Nepal, New Caledonia, Niue, Norfolk I, Palau,
    Papua New Guinea, the Philippines, Pitcairn, Qatar, Réunion, St
    Pierre & Miquelon, Samoa, Saudi Arabia, Seychelles, Singapore,
    Solomon Is, Tokelau, Tuvalu, Wake, Vanuatu, Wallis & Futuna, and
    Xinjiang; for 20-minute daylight saving time in Ghana before 1943;
    for half-hour daylight saving time in Belize before 1944 and in
    the Dominican Republic before 1975; and for Canary Islands before
    1946, for Guinea-Bissau before 1975, for Iceland before 1969, for
    Indian Summer Time before 1942, for Indonesia before around 1964,
    for Kenya before 1960, for Liberia before 1973, for Madeira before
    1967, for Namibia before 1943, for the Netherlands in 1937-9, for
    Pakistan before 1971, for Western Sahara before 1977, and for
    Zaporozhye in 1880-1924.

    For Alaska time from 1900 through 1967, instead of "CAT" use the
    abbreviation "AST", the abbreviation commonly used at the time
    (Atlantic Standard Time had not been standardized yet).  Use "AWT"
    and "APT" instead of the invented abbreviations "CAWT" and "CAPT".

Hi Iana Team,

where can I find some explanation why in 2017a format changed from AST to +03?
and the same happened in a lot of timezones (letter abbreviations changed to +03, +02, etc.)

// 2016j
Zone Asia/Riyadh 3:06:52 - LMT 1947 Mar 14
3:00 - AST

//  2017a
Zone Asia/Riyadh 3:06:52 - LMT 1947 Mar 14
3:00 - +03

Thanks & Regards,
Elena.