It’s easy to convert from base ten to base one thousand.
Our everyday counting system really is base ten, as long as we ignore those oh so convenient commas and that oh so natural (or so we think) and convenient way we say numbers and write them long hand.
Take 656,004,889. Making the base explicit we have:656,004,889ten. Pronounced ‘six hundred fifty-six million four thousand eight hundred eighty-nine’.
To convert it to base one thousand, just rewrite it as this three digit number:
one-thousand. Pronounced ‘six hundred fifty-six million four thousand eight hundred eighty-nine’ i.e. exactly the same. So you can’t tell whether the number is base one thousand or base ten from the sound of it when pronounced as a normal number. Read as digits it is ‘six hundred fifty-six four eight hundred eighty-nine’ or, just possibly, ‘six hundred fifty-six, four, eight hundred eighty-nine’. Three digits either way.
The numerals I’ve used are what I call ‘compound numerals’. In base one thousand they run from  to . Thus there’s no need to find a thousand symbols.
You can ditch the square brackets if you add spaces or some other separator between the compound numerals, e.g. 656 004 889one-thousand or 656-004-889one-thousand. Looks a bit like a telephone number, especially if you don’t hear it pronounced (like a normal base one thousand number). So you could write a base one thousand number either of these ways and people will likely think they are phone numbers.
What about using commas as separators? That would confuse people who have seen commas used as optional indicators showing how the digits could be divided into groups of three. Technically, though, you could do that. So to convert 656,004,889ten into base 1000 you *could* (it would be highly confusing, if not downright misleading) simply change the ‘ten’ on the end to ‘one-thousand’. 656,004,889ten converted to base one thousand would be: 656,004,889one-thousand. Ditching the labels indicating which base is which would allow you, sort of, to write: ‘656,004,889 in base one thousand is 656,004,889’.
Crazy, or crooked. But it shows how our counting system, as used in everyday life is a blend of base ten and base a thousand, and in a way is more base a thousand than base ten. It’s pure base ten when no commas are used: 656004889 which you often see in computer code, as long as no one pronounces it as a normal number.
Ten thousand is a myriad. Not a lot of people know that. 100,000 has no name (in English). Wikipedia says: ‘In India, Pakistan and South Asia, one hundred thousand is called a lakh, and is written as 1,00,000.’
On the whole, in English there are names only for the powers of a thousand except for numbers smaller than a thousand. Even ‘myriad’ is essentially never used nowadays to mean exactly ten thousand, but rather only to mean ‘a great number of’ as in ‘myriad raindrops fall during a rainstorm’.
Likewise, it’s easy to convert from binary to octal, but to keep this article accessible to ‘the million’ on this planet who might be interested, I’ll leave that for another article.